[mutter/wayland] Update xdg-shell.xml

[Jasper St. Pierre <jstpierre src gnome org>]

commit 40b1e7312d1dce8d71bc685622ca4909cb9c18cc
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Thu Nov 21 13:00:37 2013 -0500

    Update xdg-shell.xml

 protocol/xdg-shell.xml |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/protocol/xdg-shell.xml b/protocol/xdg-shell.xml
index 6765f98..e1249b0 100644
--- a/protocol/xdg-shell.xml
+++ b/protocol/xdg-shell.xml
@@ -121,7 +121,7 @@
        unmapped if the parent is unmapped too. They should not appear
        on task bars and alt+tab.
       </description>
-      <arg name="parent" type="object" interface="wl_surface"/>
+      <arg name="parent" type="object" interface="wl_surface" allow-null="true"/>
     </request>
 
     <request name="set_title">