Clear transposition table on "ucinewgame"

It seems the standard behaviour as implemented
in most engines although UCI protocol does not
specify what to do upon "ucinewgame" command.

No functional change.

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

src/uci.cpp

@@ -25,6 +25,7 @@
 #include "position.h"
 #include "search.h"
 #include "thread.h"
+#include "tt.h"
 #include "ucioption.h"
 
 using namespace std;
@@ -93,7 +94,7 @@ void uci_loop(const string& args) {
           go(pos, is);
 
       else if (token == "ucinewgame")
-      { /* Avoid returning "Unknown command" */ }
+          TT.clear();
 
       else if (token == "isready")
           cout << "readyok" << endl;