wayland: Put interface version defines in a common place

So far only one, but put it somewhere all files can see it.

https://bugzilla.gnome.org/show_bug.cgi?id=746141

gdk/wayland/gdkwindow-wayland.c

@@ -37,8 +37,6 @@
 #include <string.h>
 #include <errno.h>
 
-#define WL_SURFACE_HAS_BUFFER_SCALE 3
-
 #define WINDOW_IS_TOPLEVEL_OR_FOREIGN(window) \
   (GDK_WINDOW_TYPE (window) != GDK_WINDOW_CHILD &&   \
    GDK_WINDOW_TYPE (window) != GDK_WINDOW_OFFSCREEN)