protocol: fix DTD warning in weston-desktop-shell

The DTD requires elements in certain order, scanner itself doesn't.
Anyway, fix the warning with a simple reordering to match how all other
protocols are written.

Fixes:
protocol:107: element request: validity error : Element request content
does not follow the DTD, expecting (description? , arg*), got (arg
description )

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

protocol/weston-desktop-shell.xml

@@ -105,12 +105,13 @@
     </enum>
 
     <request name="set_panel_position">
-      <arg name="position" type="uint"/>
       <description summary="set panel position">
         Tell the shell which side of the screen the panel is
         located. This is so that new windows do not overlap the panel
         and maximized windows maximize properly.
       </description>
+
+      <arg name="position" type="uint"/>
     </request>
 
   </interface>