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/piece.cpp

@@ -38,7 +38,7 @@ static const string PieceChars(" pnbrqk PNBRQK");
 
 char piece_type_to_char(PieceType pt, bool upcase) {
 
-  return PieceChars[pt + upcase * 7];
+  return PieceChars[pt + int(upcase) * 7];
 }
 
 PieceType piece_type_from_char(char c) {