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/psqt.cpp

@@ -24,6 +24,7 @@
 #include "bitboard.h"
 #include "types.h"
 
+namespace Stockfish {
 
 namespace
 {
@@ -126,3 +127,5 @@ void init() {
 }
 
 } // namespace PSQT
+
+} // namespace Stockfish