Add `Cargo.lock` to git tracking #3140

Job	Run time
build GTK binaries with gvsbuild	13s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	2m 38s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	3m 2s
build gtk-rs on Windows (gio, true, --features v2_74)	5m 39s
build gtk-rs on Windows (glib, true, --features v2_74)	2m 49s
build gtk-rs on Windows (glib-build-tools, false)	41s
build gtk-rs on Windows (graphene, false)	1m 48s
build gtk-rs on Windows (pango, true, --features v1_50)	1m 49s
build gtk-rs on Windows (pangocairo, true)	3m 32s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 16s
	24m 27s

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