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/evaluate.cpp

@@ -54,7 +54,9 @@
 
 
 using namespace std;
-using namespace Eval::NNUE;
+using namespace Stockfish::Eval::NNUE;
+
+namespace Stockfish {
 
 namespace Eval {
 
@@ -1146,3 +1148,5 @@ std::string Eval::trace(const Position& pos) {
 
   return ss.str();
 }
+
+} // namespace Stockfish