glib-macros: Mark property getters as #[must_use] #3059

Job	Run time
build GTK binaries with gvsbuild	29s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	5m 49s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	2m 25s
build gtk-rs on Windows (gio, true, --features v2_74)	10m 15s
build gtk-rs on Windows (glib, true, --features v2_74)	3m 44s
build gtk-rs on Windows (glib-build-tools, false)	1m 11s
build gtk-rs on Windows (graphene, false)	2m 34s
build gtk-rs on Windows (pango, true, --features v1_50)	3m 27s
build gtk-rs on Windows (pangocairo, true)	3m 52s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	3m 4s
	36m 50s

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