MovePicker: take advantage of threat move for ordering

If the null move was refuted by a capture then give a
bonus if we move away the captured piece.

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

src/movepick.h

@@ -83,7 +83,7 @@ private:
   int find_best_index(Bitboard* squares, int values[]);
 
   const Position& pos;
-  Move ttMove, mateKiller, killer1, killer2;
+  Move ttMove, mateKiller, killer1, killer2, threatMove;
   Bitboard pinned, dc;
   MoveStack moves[256], badCaptures[64];
   bool pvNode;