sync: make padding explicit

Add explicit align-padding for struct SYSTEMCOUNTER:
Added 4-byte align-pad at the end.
This is field "p" in the spec of SYSTEMCOUNTER at:
  http://cgit.freedesktop.org/xorg/proto/xextproto/tree/specs/sync.xml?id=xextproto-7.3.0#n983



Signed-off-by: Christian Linhart <chris@demorecorder.com>
Reviewed-by: Peter Harris <pharris@opentext.com>

src/sync.xml

@@ -53,6 +53,7 @@ for licensing information.
     <list type="char" name="name">
       <fieldref>name_len</fieldref>
     </list>
+    <pad align="4" />
   </struct>
 
   <struct name="TRIGGER">