Rover: always build with MAVLink2 headers

APMrover2/GCS_Mavlink.cpp

@@ -1385,11 +1385,9 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
         send_autopilot_version(FIRMWARE_VERSION);
         break;
 
-#if MAVLINK_PROTOCOL_VERSION >= 2
     case MAVLINK_MSG_ID_SETUP_SIGNING:
         handle_setup_signing(msg);
         break;
-#endif
     } // end switch
 } // end handle mavlink