wayland: Don't provide gsetting if dconf is not available

This makes gtk+ fall back to reading ~/.config/gtk-3.0/settings.ini
on systems with Wayland, but without dconf (do those exist?).

https://bugzilla.gnome.org/show_bug.cgi?id=790201

gdk/wayland/gdkscreen-wayland.c

@@ -657,6 +657,9 @@ gdk_wayland_screen_get_setting (GdkScreen   *screen,
 
   g_return_val_if_fail (GDK_IS_SCREEN (screen), FALSE);
 
+  if (g_settings_schema_source_get_default () == NULL)
+    return FALSE;
+
   entry = find_translation_entry_by_setting (name);
   if (entry != NULL)
     {