[notifier] add strings for newly added OT_CHANGED flags in FlagToString (#3335)

src/core/common/notifier.cpp

@@ -332,6 +332,14 @@ const char *Notifier::FlagToString(otChangedFlags aFlag) const
         retval = "ChanMask";
         break;
 
+    case OT_CHANGED_BORDER_AGENT_STATE:
+        retval = "BorderAgentState";
+        break;
+
+    case OT_CHANGED_THREAD_NETIF_STATE:
+        retval = "NetifState";
+        break;
+
     default:
         break;
     }