Fixup.

coupler/keep_alive_subscriber.h

@@ -66,7 +66,7 @@ static void dataChangeNotificationCallback(UA_Server *server, UA_UInt32 monitore
         // it is completely normal to have same multicast group keep alive
         // messages from different couplers thus filter and leave ones we care for
         bool is_coupler_id_interesting = false;
-        for (int i = 0; i < HEART_BEAT_ID_LIST_COUNT; i++) {
+        for (unsigned int i = 0; i < HEART_BEAT_ID_LIST_COUNT; i++) {
           if (coupler_id == HEART_BEAT_ID_LIST[i]){
             // we care for this ID
             is_coupler_id_interesting = true;