Fix build with MSVC 2013

Also add an assert hinted by MSVC code analysis tool.

No functional change.

src/pawns.cpp

@@ -17,6 +17,7 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <algorithm>
 #include <cassert>
 
 #include "bitboard.h"