Enable SNTP in zephyr example

src/net.c

@@ -198,7 +198,7 @@ struct mg_connection *mg_listen(struct mg_mgr *mgr, const char *url,
     c->fn = fn;
     c->fn_data = fn_data;
     mg_call(c, MG_EV_OPEN, NULL);
-    MG_DEBUG(("%lu %s", c->id, url));
+    MG_DEBUG(("%lu %s %p", c->id, url, c->fd));
   }
   return c;
 }