x11-toolkits/gtk-layer-shell: silence gtk3 warning after 2b1cd908bf52

$ daemon -f swaync
$ notify-send foo bar

• (swaync:1234): WARNING **: gtk-layer-shell v0.6.0 may not work on GTK v3.24.29. If you experience crashes, check https://github.com/wmww/gtk-layer-shell/blob/master/compatibility.md

(cherry picked from commit 0706634a2903f60f6e1567663993136daf288d1b)