Add Stockfish namespace.

fixes #3350 and is a small cleanup that might make it easier to use SF
in separate projects, like a NNUE trainer or similar.

closes https://github.com/official-stockfish/Stockfish/pull/3370

No functional change.

src/bitboard.cpp

@@ -22,6 +22,8 @@
 #include "bitboard.h"
 #include "misc.h"
 
+namespace Stockfish {
+
 uint8_t PopCnt16[1 << 16];
 uint8_t SquareDistance[SQUARE_NB][SQUARE_NB];
 
@@ -42,7 +44,6 @@ namespace {
 
 }
 
-
 /// safe_destination() returns the bitboard of target square for the given step
 /// from the given square. If the step is off the board, returns empty bitboard.
 
@@ -111,7 +112,6 @@ void Bitboards::init() {
   }
 }
 
-
 namespace {
 
   Bitboard sliding_attack(PieceType pt, Square sq, Bitboard occupied) {
@@ -211,3 +211,5 @@ namespace {
     }
   }
 }
+
+} // namespace Stockfish