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/layers/affine_transform.h

@@ -24,7 +24,7 @@
 #include <iostream>
 #include "../nnue_common.h"
 
-namespace Eval::NNUE::Layers {
+namespace Stockfish::Eval::NNUE::Layers {
 
   // Affine transformation layer
   template <typename PreviousLayer, IndexType OutputDimensions>
@@ -459,6 +459,6 @@ namespace Eval::NNUE::Layers {
 #endif
   };
 
-}  // namespace Eval::NNUE::Layers
+}  // namespace Stockfish::Eval::NNUE::Layers
 
 #endif // #ifndef NNUE_LAYERS_AFFINE_TRANSFORM_H_INCLUDED