Build fail with gcc 9

### Expected behavior

Compilation completes successfully.

### Actual behavior

Compilation failure:

```
$DEBUG_LEVEL is 1
Makefile:168: Warning: Compiling in debug mode. Don't use the resulting binary in production
GEN util/build_version.cc
$DEBUG_LEVEL is 1
Makefile:168: Warning: Compiling in debug mode. Don't use the resulting binary in production
GEN util/build_version.cc
CC cache/clock_cache.o
CC cache/lru_cache.o
CC cache/sharded_cache.o
CC db/builder.o
In file included from ./db/memtable.h:21,
from ./db/memtable_list.h:17,
from ./db/column_family.h:17,
from ./db/version_set.h:31,
from ./db/compaction.h:11,
from ./db/compaction_iterator.h:13,
from db/builder.cc:16:
./db/version_edit.h: In constructor ‘rocksdb::FdWithKeyRange::FdWithKeyRange(rocksdb::FileDescriptor, rocksdb::Slice, rocksdb::Slice, rocksdb::FileMetaData*)’:
./db/version_edit.h:178:33: error: implicitly-declared ‘constexpr rocksdb::FileDescriptor::FileDescriptor(const rocksdb::FileDescriptor&)’ is deprecated [-Werror=deprecated-copy]
178 | largest_key(_largest_key) {}
| ^
./db/version_edit.h:55:19: note: because ‘rocksdb::FileDescriptor’ has user-provided ‘rocksdb::FileDescriptor& rocksdb::FileDescriptor::operator=(const rocksdb::FileDescriptor&)’
55 | FileDescriptor& operator=(const FileDescriptor& fd) {
| ^~~~~~~~
./db/version_edit.h: In instantiation of ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, _U2&&) [with _U1 = int&; _U2 = rocksdb::FileMetaData; typename std::enable_if<(std::_PCC::_MoveConstructiblePair<_U1, _U2>() && std::_PCC::_ImplicitlyMoveConvertiblePair<_U1, _U2>()), bool>::type = true; _T1 = int; _T2 = rocksdb::FileMetaData]’:
/usr/include/c++/9/ext/new_allocator.h:147:4: required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair; _Args = {int&, rocksdb::FileMetaData}; _Tp = std::pair]’
/usr/include/c++/9/bits/alloc_traits.h:484:4: required from ‘static void std::allocator_traits >::construct(std::allocator_traits >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair; _Args = {int&, rocksdb::FileMetaData}; _Tp = std::pair; std::allocator_traits >::allocator_type = std::allocator >]’
/usr/include/c++/9/bits/vector.tcc:115:30: required from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {int&, rocksdb::FileMetaData}; _Tp = std::pair; _Alloc = std::allocator >]’
./db/version_edit.h:253:48: required from here
./db/version_edit.h:86:8: error: implicitly-declared ‘constexpr rocksdb::FileDescriptor::FileDescriptor(const rocksdb::FileDescriptor&)’ is deprecated [-Werror=deprecated-copy]
86 | struct FileMetaData {
| ^~~~~~~~~~~~
./db/version_edit.h:55:19: note: because ‘rocksdb::FileDescriptor’ has user-provided ‘rocksdb::FileDescriptor& rocksdb::FileDescriptor::operator=(const rocksdb::FileDescriptor&)’
55 | FileDescriptor& operator=(const FileDescriptor& fd) {
| ^~~~~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:64,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/string:40,
from ./db/builder.h:9,
from db/builder.cc:10:
/usr/include/c++/9/bits/stl_pair.h:342:64: note: synthesized method ‘rocksdb::FileMetaData::FileMetaData(rocksdb::FileMetaData&&)’ first required here
342 | : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y)) { }
| ^
In file included from ./db/memtable.h:21,
from ./db/memtable_list.h:17,
from ./db/column_family.h:17,
from ./db/version_set.h:31,
from ./db/compaction.h:11,
from ./db/compaction_iterator.h:13,
from db/builder.cc:16:
./db/version_edit.h: In instantiation of ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, const _T2&) [with _U1 = int&; typename std::enable_if::_MoveCopyPair(), bool>::type = true; _T1 = int; _T2 = rocksdb::FileMetaData]’:
/usr/include/c++/9/ext/new_allocator.h:147:4: required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair; _Args = {int&, const rocksdb::FileMetaData&}; _Tp = std::pair]’
/usr/include/c++/9/bits/alloc_traits.h:484:4: required from ‘static void std::allocator_traits >::construct(std::allocator_traits >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair; _Args = {int&, const rocksdb::FileMetaData&}; _Tp = std::pair; std::allocator_traits >::allocator_type = std::allocator >]’
/usr/include/c++/9/bits/vector.tcc:115:30: required from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {int&, const rocksdb::FileMetaData&}; _Tp = std::pair; _Alloc = std::allocator >]’
./db/version_edit.h:258:37: required from here
./db/version_edit.h:86:8: error: implicitly-declared ‘constexpr rocksdb::FileDescriptor::FileDescriptor(const rocksdb::FileDescriptor&)’ is deprecated [-Werror=deprecated-copy]
86 | struct FileMetaData {
| ^~~~~~~~~~~~
./db/version_edit.h:55:19: note: because ‘rocksdb::FileDescriptor’ has user-provided ‘rocksdb::FileDescriptor& rocksdb::FileDescriptor::operator=(const rocksdb::FileDescriptor&)’
55 | FileDescriptor& operator=(const FileDescriptor& fd) {
| ^~~~~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:64,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/string:40,
from ./db/builder.h:9,
from db/builder.cc:10:
/usr/include/c++/9/bits/stl_pair.h:312:51: note: synthesized method ‘rocksdb::FileMetaData::FileMetaData(const rocksdb::FileMetaData&)’ first required here
312 | : first(std::forward<_U1>(__x)), second(__y) { }
| ^
cc1plus: all warnings being treated as errors
make: *** [Makefile:1953: db/builder.o] Error 1
```

### Steps to reproduce the behavior

```
% gcc --version :(
gcc (GCC) 9.1.1 20190503 (Red Hat 9.1.1-1)
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
% uname -r
5.0.13-300.fc30.x86_64
```
Simple make command can reproduce the error.