Fix a warning with __popcnt64() intrinsics

Returns an int64_t while we want a simple int.

This occurs only when compiling with MSVC on a 64 bit platform.

No functional change.

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

src/bitcount.h

@@ -88,7 +88,7 @@ inline int count_1s<CNT_POPCNT>(Bitboard b) {
 #elif defined(_MSC_VER) && defined(__INTEL_COMPILER)
   return _mm_popcnt_u64(b);
 #elif defined(_MSC_VER)
-  return __popcnt64(b);
+  return (int)__popcnt64(b);
 #elif defined(__GNUC__)
   unsigned long ret;
   __asm__("popcnt %1, %0" : "=r" (ret) : "r" (b));