gtk.cfg: Fix gtk_widget_destroy definition and usage #19757

Job	Run time
build_cmake (ubuntu:24.04, true, true)	2m 32s
build_make (ubuntu:25.10)	3m 29s
build_make (alpine:3.23)	5m 53s
build_cmake (ubuntu:25.10, true, true)	2m 42s
build_cmake (alpine:3.23, false, false)	4m 22s
build_make (ubuntu:24.04)	2m 38s
	21m 36s

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