window: GtkPlug is conditionally supported on X11 only
window: GtkPlug is conditionally supported on X11 only

We cannot do a type check on it if we don't have the X11 backend
compiled in.

https://bugzilla.gnome.org/show_bug.cgi?id=739885

jadahl/gtk,ahodesuka/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,ahodesuka/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,Lyude/gtk-,grubersjoe/adwaita,Lyude/gtk-,jigpu/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,jigpu/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,Lyude/gtk-,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,Lyude/gtk-,jadahl/gtk,Adamovskiy/gtk,Lyude/gtk-,chergert/gtk,Lyude/gtk-,ahodesuka/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,davidgumberg/gtk