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/position.h

@@ -31,6 +31,7 @@
 
 #include "nnue/nnue_accumulator.h"
 
+namespace Stockfish {
 
 /// StateInfo struct stores information needed to restore a Position object to
 /// its previous state when we retract a move. Whenever a move is made on the
@@ -423,4 +424,6 @@ inline StateInfo* Position::state() const {
   return st;
 }
 
+} // namespace Stockfish
+
 #endif // #ifndef POSITION_H_INCLUDED