win32: add gwin32compat.h, for utf8-renaming compatibility defines

To avoid -Wmissing-prototype warnings, we need to prototype both the
original and the _utf8 versions of all of the functions that have had
_utf8-renaming on Windows. But duplicating all the prototypes is ugly,
so rather than doing them "in-place", move them all to a new header
file just for that.

https://bugzilla.gnome.org/show_bug.cgi?id=688109