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/evaluate_nnue.cpp

@@ -29,7 +29,7 @@
 
 #include "evaluate_nnue.h"
 
-namespace Eval::NNUE {
+namespace Stockfish::Eval::NNUE {
 
   // Input feature converter
   LargePagePtr<FeatureTransformer> feature_transformer;
@@ -141,4 +141,4 @@ namespace Eval::NNUE {
     return ReadParameters(stream);
   }
 
-} // namespace Eval::NNUE
+} // namespace Stockfish::Eval::NNUE