Fix a shift overflow warning

Visual Studio 11 is worried that shift result could
overflow an integer, this is impossible becuase max
value of the shift is 4, but compiler cannot know it.

No functional change.

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

src/position.cpp

@@ -1520,7 +1520,7 @@ void Position::init() {
       Bitboard b = cr;
       while (b)
       {
-          Key k = zobCastle[1 << pop_1st_bit(&b)];
+          Key k = zobCastle[1ULL << pop_1st_bit(&b)];
           zobCastle[cr] ^= k ? k : rk.rand<Key>();
       }
   }