update example

src/sntp.h

@@ -6,3 +6,5 @@ struct mg_connection *mg_sntp_connect(struct mg_mgr *mgr, const char *url,
                                       mg_event_handler_t fn, void *fn_data);
 void mg_sntp_request(struct mg_connection *c);
 int64_t mg_sntp_parse(const unsigned char *buf, size_t len);
+
+uint64_t mg_now(void);     // Return milliseconds since Epoch