redefinition of ‘struct extent_hooks_s’

Building on Archlinux gives me the following error:
```
Building CXX object CMakeFiles/folly_base.dir/folly/experimental/JemallocHugePageAllocator.cpp.o
/home/ftiasch/Documents/folly/src/folly-2018.09.24.00/folly/experimental/JemallocHugePageAllocator.cpp:52:8: error: redefinition of ‘struct extent_hooks_s’
struct extent_hooks_s {
^~~~~~~~~~~~~~
In file included from /home/ftiasch/Documents/folly/src/folly-2018.09.24.00/folly/memory/Malloc.h:32,
from /home/ftiasch/Documents/folly/src/folly-2018.09.24.00/folly/experimental/JemallocHugePageAllocator.h:22,
from /home/ftiasch/Documents/folly/src/folly-2018.09.24.00/folly/experimental/JemallocHugePageAllocator.cpp:17:
/usr/include/jemalloc/jemalloc.h:339:8: note: previous definition of ‘struct extent_hooks_s’
struct extent_hooks_s {
^~~~~~~~~~~~~~
```

Is the jemalloc installed too new?

Start with folly/experimental/JemallocHugePageAllocator.cpp and compare its extent_hooks_s definition with the declaration included through folly/memory/Malloc.h and /usr/include/jemalloc/jemalloc.h. Reproduce the Arch Linux CMake build, then verify that the allocator source compiles without the redefinition error.

Written by the indexing model from the issue text.