Add missing S_ISREG definitions for VS2022

Philip Withnall requested to merge pwithnall/xdgmime:s-isreg into master

Otherwise building with VS2022 fails with:

Creating library D:/temp/29/.sw/out/323969/gnome.glib.gio-2.71.0.lib and object D:/temp/29/.sw/out/323969/gnome.glib.gio-2.71.0.exp
xdgmime.c.4b279509.obj : error LNK2019: unresolved external symbol S_ISREG referenced in function _gio_xdg_get_mime_type_for_file
xdgmimecache.c.79af418f.obj : error LNK2001: unresolved external symbol S_ISREG
D:\temp\29\.sw\out\323969\gnome.glib.gio-2.71.0.dll : fatal error LNK1120: 1 unresolved externals


Signed-off-by: Philip Withnall

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