GDK: fix build when the X11 backend is disabled
GDK: fix build when the X11 backend is disabled

https://bugzilla.gnome.org/show_bug.cgi?id=695710

Distrotech/clutter,ebassi/clutter,jigpu/clutter,GNOME/clutter,GNOME/clutter,collects/clutter,jigpu/clutter,djdeath/clutter,collects/clutter,ebassi/clutter,ebassi/clutter,jigpu/clutter,jigpu/clutter,heysion/clutter-clone,Distrotech/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,Distrotech/clutter,collects/clutter,heysion/clutter-clone,Distrotech/clutter,heysion/clutter-clone,heysion/clutter-clone,heysion/clutter-clone,ebassi/clutter,GNOME/clutter,collects/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,ebassi/clutter,Distrotech/clutter,GNOME/clutter,collects/clutter,GNOME/clutter