axtls: Add a TODO to a potential blocking call with no timeout

lib/vtls/axtls.c

@@ -481,6 +481,7 @@ Curl_axtls_connect(struct connectdata *conn,
       return map_error_to_curl(ssl_fcn_return);
     }
     usleep(10000);
+    /* TODO: check for timeout here as this could hang indefinitely otherwise */
   }
   infof (conn->data, "handshake completed successfully\n");