Summary:
Fixing cast-function-type which is appears during the following build:
```bash
cmake ..  -DFAIL_ON_WARNINGS=ON -DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc -DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-g++ -DCMAKE_SYSTEM_NAME=Windows
make rocksdb
```
Here is the log:
```
/home/leshiy/Work/rocksdb/port/win/env_win.cc: In constructor ‘rocksdb::port::WinClock::WinClock()’:
/home/leshiy/Work/rocksdb/port/win/env_win.cc:92:9: error: cast between incompatible function types from ‘FARPROC’ {aka ‘long long int (*)()’} to ‘rocksdb::port::WinClock::FnGetSystemTimePreciseAsFileTime’ {aka ‘void (*)(_FILETIME*)’} [-Werror=cast-function-type]
   92 |         (FnGetSystemTimePreciseAsFileTime)GetProcAddress(
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   93 |             module, "GetSystemTimePreciseAsFileTime");
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [CMakeFiles/rocksdb.dir/build.make:4337: CMakeFiles/rocksdb.dir/port/win/env_win.cc.obj] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/rocksdb.dir/all] Error 2
make: *** [Makefile:91: all] Error 2
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8230

Reviewed By: jay-zhuang

Differential Revision: D28000215

Pulled By: mrambacher

fbshipit-source-id: 874782cf48f70470e3fbd9097585bf42e810ca61