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/gdkprivate-wayland.h

@@ -40,6 +40,8 @@
 
 #include "config.h"
 
+#define WL_SURFACE_HAS_BUFFER_SCALE 3
+
 #define GDK_WINDOW_IS_WAYLAND(win)    (GDK_IS_WINDOW_IMPL_WAYLAND (((GdkWindow *)win)->impl))
 
 GdkKeymap *_gdk_wayland_keymap_new (void);