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.h

@@ -25,7 +25,7 @@
 
 #include <memory>
 
-namespace Eval::NNUE {
+namespace Stockfish::Eval::NNUE {
 
   // Hash value of evaluation function structure
   constexpr std::uint32_t kHashValue =
@@ -54,6 +54,6 @@ namespace Eval::NNUE {
   template <typename T>
   using LargePagePtr = std::unique_ptr<T, LargePageDeleter<T>>;
 
-}  // namespace Eval::NNUE
+}  // namespace Stockfish::Eval::NNUE
 
 #endif // #ifndef NNUE_EVALUATE_NNUE_H_INCLUDED