gtkiconcache: use _fstat64 under 64bit mingw to match GStatBuf

Merged Christoph Reiter requested to merge creiter/gtk:mingw-iconcache-use-correct-fstat into gtk-3-24

With the switch to meson we no longer built with _FILE_OFFSET_BITS which results in fstat being the 32 bit variant, not matching GStatBuf, which is _stat64 on 64bit mingw builds.

This makes things use _fstat64 in this case, to match GStatBuf.

See !1056 (closed)

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