Fix some warnings under +w1 HP-UX compile

This is the world's fussiest compiler with +w1

Warnings reported by Richard Lloyd.

No functional change.

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

src/evaluate.cpp

@@ -961,7 +961,7 @@ namespace {
             qsq = relative_square(Us, make_square(square_file(s), RANK_8));
             d =  square_distance(s, qsq)
                - square_distance(theirKingSq, qsq)
-               + (Us != pos.side_to_move());
+               + int(Us != pos.side_to_move());
 
             if (d < 0)
             {