Use uint64_t for uptime in millis, not int64_t

src/sock.c

@@ -532,7 +532,7 @@ static void connect_conn(struct mg_connection *c) {
 
 void mg_mgr_poll(struct mg_mgr *mgr, int ms) {
   struct mg_connection *c, *tmp;
-  int64_t now;
+  uint64_t now;
 
   mg_iotest(mgr, ms);
   now = mg_millis();