build: Always compile with _FILE_OFFSET_BITS=64 on Windows (MinGW)

* cmake/modules/G10CompilerSettings.cmake: Add -D_FILE_OFFSET_BITS=64
for MinGW.

This makes gpgmepp compatible with gpgol which we also compile with
_FILE_OFFSET_BITS=64.

• GnuPG-bug-id: T7262