Stop is not an unknown command

If GUI sends stop while we are waiting for
a command do not reply with a silly:

Unknown command: stop

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/uci.cpp

@@ -72,6 +72,9 @@ void uci_loop() {
       if (token == "quit")
           quit = true;
 
+      else if (token == "stop")
+      { /* avoid to reply "Unknown command: stop" */ }
+
       else if (token == "go")
           quit = !go(pos, is);