headers: Guard _fstat, _fstati64, _stat, _stati64, _wstat and _wstati64 for CRT build

The mingw-w64 runtime should not call these functions as they are
conditionally defined based on application compile-time macro
_USE_32BIT_TIME_T.

So hide all these functions when building the mingw-w64 runtime to prevent
ABI issues.

Signed-off-by: Martin Storsjö <martin@martin.st>