HPCC-8274 - Fix compile issue introduced by gh-3681

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>

system/mp/mpcomm.cpp

@@ -180,6 +180,7 @@ struct MultiPacketHeader
         out.append(",idx=").append(idx);
         out.append(",numparts=").append(numparts);
         out.append(",total=").append(total);
+        return out;
     }
 };