Don't use _pext_u64() directly

This intrinsic to call BMI2 PEXT instruction is
defined in immintrin.h. This header should be
included only when USE_PEXT is defined, otherwise
we define _pext_u64 as 0 forcing a nop.

But under some mingw platforms, even if we don't
include the header, immintrin.h gets included
anyhow through an include chain that starts with
STL <algorithm> header. So we end up both defining
_pext_u64 function and at the same time defining
_pext_u64 as 0 leading to a compile error.

The correct solution is of not using _pext_u64 directly.

This patch fixes a compile error with some mingw64
package when compiling with x86-64.

No functional change.

src/bitboard.h

@@ -238,7 +238,7 @@ FORCE_INLINE unsigned magic_index(Square s, Bitboard occupied) {
   unsigned* const Shifts = Pt == ROOK ? RookShifts : BishopShifts;
 
   if (HasPext)
-      return unsigned(_pext_u64(occupied, Masks[s]));
+      return unsigned(pext(occupied, Masks[s]));
 
   if (Is64Bit)
       return unsigned(((occupied & Masks[s]) * Magics[s]) >> Shifts[s]);