That warning has been widely reported for countless packages and should be resolved by the app developer. Still, to keep people happy until then, it will be silenced by default in GTK+ 3.22.1 (accidentally got omitted in 3.22.0)
https://git.gnome.org/browse/gtk+/commit/?id=54fdcb3ffac3383432b379f3e16e8cb0086b8101
