gtk3: Enable Wayland backend

Because we can.

> This switch enables the Wayland GDK backend. Replace with
> --disable-wayland-backend if you do not have Wayland-1.14.0 and
> wayland-protocols-1.12 or you do not wish to build GNOME with Wayland
> support. Other changes may be needed to allow the build to complete
> without the wayland-backend enabled.

[1] http://www.linuxfromscratch.org/blfs/view/svn/x/gtk3.html

gtk3.be0

@@ -56,6 +56,7 @@ SRCURL[0]="https://ftp.gnome.org/pub/gnome/sources/gtk+/${PKGVERSION[2]}/gtk+-${
 mee_configure() {
     bee_configure \
         --enable-introspection \
+        --enable-wayland-backend \
         --enable-xinerama
 }