x11-toolkits/gtk-layer-shell: silence gtk3 warning after 89fb405a8674

$ waybar
[...]

• (waybar:69072): WARNING **: gtk-layer-shell v0.7.0 may not work on GTK v3.24.33. If you experience crashes, check https://github.com/wmww/gtk-layer-shell/blob/master/compatibility.md

PR: 262419
(cherry picked from commit 9965cf5dea7981283369877a7b149ed67f3163ca)