gio: Only provide UNIX mount compat API on UNIX platforms #4079

Job	Run time
build GTK binaries with gvsbuild	28s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	2m 8s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	2m 18s
build gtk-rs on Windows (gio, true, --features v2_80)	3m 50s
build gtk-rs on Windows (glib, true, --features v2_80)	3m 20s
build gtk-rs on Windows (glib-build-tools, false)	2m 43s
build gtk-rs on Windows (graphene, false)	1m 13s
build gtk-rs on Windows (pango, true, --features v1_52)	1m 35s
build gtk-rs on Windows (pangocairo, true)	2m 2s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 13s
	21m 50s

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