Don't need to wait after a "ponderhit"

It is enough to wake up main thread. This is
a better fix than d033d5e06a.

No functional change.

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

src/uci.cpp

@@ -87,7 +87,7 @@ void uci_loop(const string& args) {
           if (Search::Signals.stopOnPonderhit)
           {
               Search::Signals.stop = true;
-              Threads.wait_for_search_finished(); // Wake up if is sleeping
+              Threads.main_thread()->wake_up(); // Could be sleeping
           }
       }