xinput: struct AddMaster: add align-pad at end

reasons:
* length of the struct is a multiple of 4 according to spec.
* list of char may not end at a 4-byte aligned position.

see:
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XI2proto.txt#n1224
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XI2proto.h#n454
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XI2proto.h#n458
(file-positions may become wrong if the spec or header change)

src/xinput.xml

@@ -1200,6 +1200,7 @@ authorization from the authors.
         <list type="char" name="name">
             <fieldref>name_len</fieldref>
         </list>
+        <pad align="4" />
     </struct>
 
     <struct name="RemoveMaster">