glib-macros: Mark property getters as #[must_use] #3058

Job	Run time
build GTK binaries with gvsbuild	32s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	3m 34s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	3m 2s
build gtk-rs on Windows (gio, true, --features v2_74)	4m 41s
build gtk-rs on Windows (glib, true, --features v2_74)	4m 11s
build gtk-rs on Windows (glib-build-tools, false)	1m 13s
build gtk-rs on Windows (graphene, false)	3m 28s
build gtk-rs on Windows (pango, true, --features v1_50)	3m 41s
build gtk-rs on Windows (pangocairo, true)	4m 10s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 50s
	31m 22s

Provide feedback