Fix testgtk.c when building for Wayland.
Fix testgtk.c when building for Wayland.

Fix wrong usage of GDK_WINDOWING_WAYLAND instead of
GDK_WINDOWING_X11 in test/testgtk.c.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>

https://bugzilla.gnome.org/show_bug.cgi?id=737363

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