Make gdk_keymap_map_virtual_modifiers work early

It needs to ensure that we have an uptodate modmap. Bug 616401
(cherry picked from commit d5a8a3c9bc)

gdk/x11/gdkkeys-x11.c

@@ -1794,6 +1794,7 @@ gdk_keymap_map_virtual_modifiers (GdkKeymap       *keymap,
 
   keymap = GET_EFFECTIVE_KEYMAP (keymap);
   keymap_x11 = GDK_KEYMAP_X11 (keymap);
+  get_xkb (keymap_x11);
 
   retval = TRUE;