common: Fix build on non-Linux systems

Only define HAVE_WAYLAND when GDK_WINDOWING_WAYLAND is defined.

https://bugzilla.gnome.org/show_bug.cgi?id=743266

panels/common/gnome-settings-bus.h

@@ -3,6 +3,8 @@
 
 #include <gdk/gdkx.h>
 
+#ifdef GDK_WINDOWING_WAYLAND
+
 #define HAVE_WAYLAND 1
 
 static inline gboolean
@@ -10,3 +12,5 @@ gnome_settings_is_wayland (void)
 {
   return !GDK_IS_X11_DISPLAY (gdk_display_get_default ());
 }
+
+#endif /* GDK_WINDOWING_WAYLAND */