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/nnue/nnue_accumulator.h

@@ -23,7 +23,7 @@
 
 #include "nnue_architecture.h"
 
-namespace Eval::NNUE {
+namespace Stockfish::Eval::NNUE {
 
   // The accumulator of a StateInfo without parent is set to the INIT state
   enum AccumulatorState { EMPTY, COMPUTED, INIT };
@@ -35,6 +35,6 @@ namespace Eval::NNUE {
     AccumulatorState state[2];
   };
 
-}  // namespace Eval::NNUE
+}  // namespace Stockfish::Eval::NNUE
 
 #endif // NNUE_ACCUMULATOR_H_INCLUDED