xdg-shell: add missing enum attribute to set_constraint_adjustment

Closes: wayland/wayland-protocols#177


Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>

stable/xdg-shell/xdg-shell.xml

@@ -344,7 +344,7 @@
 
 	The default adjustment is none.
       </description>
-      <arg name="constraint_adjustment" type="uint"
+      <arg name="constraint_adjustment" type="uint" enum="constraint_adjustment"
 	   summary="bit mask of constraint adjustments"/>
     </request>