GCS_MAVLink: resolve gcs::send_text compiler warning

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -812,7 +812,7 @@ bool GCS_MAVLINK::set_mavlink_message_id_interval(const uint32_t mavlink_id,
 {
     const ap_message id = mavlink_id_to_ap_message_id(mavlink_id);
     if (id == MSG_LAST) {
-        gcs().send_text(MAV_SEVERITY_INFO, "No ap_message for mavlink id (%u)", mavlink_id);
+        gcs().send_text(MAV_SEVERITY_INFO, "No ap_message for mavlink id (%u)", (unsigned int)mavlink_id);
         return false;
     }
     return set_ap_message_interval(id, interval_ms);