[cpp] gencpp generates malformed C++ code for benchmarks

Trying to build our benchmarks:
```
$ cd tests/benchs && haxe build.hxml -D test=Ds -cpp bin/cpp
<...>
Error: ./src/cases/Ds.cpp: In member function ‘Dynamic cases::Ds_obj::measureRead()’:
./src/cases/Ds.cpp:219:26: error: expected primary-expression before ‘t’
HXLINE( 22) ::Dynamic t = null();
^
./src/cases/Ds.cpp:219:26: error: expected ‘}’ before ‘t’
./src/cases/Ds.cpp:219:26: error: could not convert ‘{}’ from ‘’ to ‘haxe::ds::StringMap {aka hx::ObjectPtr}’
./src/cases/Ds.cpp: At global scope:
./src/cases/Ds.cpp:223:26: error: expected primary-expression before ‘t1’
HXLINE( 23) ::Dynamic t1 = null();
^~
./src/cases/Ds.cpp:223:26: error: expected ‘}’ before ‘t1’
./src/cases/Ds.cpp:223:26: error: could not convert ‘{}’ from ‘’ to ‘haxe::ds::IntMap {aka hx::ObjectPtr}’
./src/cases/Ds.cpp:224:48: error: expected constructor, destructor, or type conversion before ‘(’ token
HXDLIN( 23) ::haxe::ds::IntMap_obj::__alloc( HX_CTX );
^
./src/cases/Ds.cpp:227:26: error: expected primary-expression before ‘t2’
HXLINE( 24) ::Dynamic t2 = null();
^~
./src/cases/Ds.cpp:227:26: error: expected ‘}’ before ‘t2’
./src/cases/Ds.cpp:227:26: error: could not convert ‘{}’ from ‘’ to ‘haxe::ds::ObjectMap {aka hx::ObjectPtr}’
./src/cases/Ds.cpp:228:51: error: expected constructor, destructor, or type conversion before ‘(’ token
HXDLIN( 24) ::haxe::ds::ObjectMap_obj::__alloc( HX_CTX );
^
./src/cases/Ds.cpp:229:15: error: expected declaration before ‘}’ token
}
^
Error: Build failed
```