Building for Win7 attempts to use a struct definition and enum value not defined until Win8

In the `WinFileSystem::AreFilesSame` function, the `GetFileInformationByHandleEx` Win32 API is invoked. It requests the `FileIdInfo` enum value and passes a pointer to a `FILE_ID_INFO` struct. These are not defined in WinBase.h if `_WIN32_WINNT < _WIN32_WINNT_WIN8` so I think the `#ifdef` at the top of this function should be changed to that.
https://github.com/facebook/rocksdb/blob/ee239df351f486a23793e637125965a3c0c3987f/port/win/env_win.cc#L918