gtk.cfg: Fix gtk_widget_destroy definition and usage #522

Job	Run time
build (ubsan, -DANALYZE_UNDEFINED=On, true, process, true)	47m 15s
build (asan, -DANALYZE_ADDRESS=On, true, process, false)	12m 39s
build (tsan, -DANALYZE_THREAD=On, false, thread, false, --executor=thread --error-exitcode=0)	11m 26s
	1h 11m 20s

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