[win32/nfs] - fix wrong struct stat size when running on win8 64

As the title says. This is an isolated bugfix to the fact that struct stat on windows expands to the 32bit version even if FILE_OFFSET_BITS=64 is set. This leads to wrong filesizes and wrong seeks after wards - resulting in non playable files if they are bigger then 4gb ...

Issue discussed and bugfix tested here:

https://github.com/sahlberg/libnfs/issues/46

I hate windows ... I hate windows ... I hate windows ... I hate windows...