gtk.cfg: Add pure annotations for more GLib functions #20200

Job	Run time
build_make (ubuntu:25.10)	4m 58s
build_make (ubuntu:24.04)	4m 15s
build_make (alpine:3.23)	5m 43s
build_cmake (alpine:3.23, false, false)	4m 6s
build_cmake (ubuntu:24.04, true, true)	5m 18s
build_cmake (ubuntu:25.10, true, true)	5m 36s
	29m 56s

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