Fix undefined reference to get_xkb()

When configuring Gtk+ with --disable-xkb, the build fails because of an
undefined reference to get_xkb().

This patch fixes this issue.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>

https://bugzilla.gnome.org/show_bug.cgi?id=739070

gdk/x11/gdkkeys-x11.c

@@ -1499,8 +1499,10 @@ gdk_x11_keymap_map_virtual_modifiers (GdkKeymap       *keymap,
   int i, j;
   gboolean retval;
 
+#ifdef HAVE_XKB
   if (KEYMAP_USE_XKB (keymap))
     get_xkb (keymap_x11);
+#endif
 
   retval = TRUE;