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/movegen.h

@@ -23,6 +23,8 @@
 
 #include "types.h"
 
+namespace Stockfish {
+
 class Position;
 
 enum GenType {
@@ -70,4 +72,6 @@ private:
   ExtMove moveList[MAX_MOVES], *last;
 };
 
+} // namespace Stockfish
+
 #endif // #ifndef MOVEGEN_H_INCLUDED