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/thread_win32_osx.h

@@ -31,6 +31,8 @@
 
 #include <pthread.h>
 
+namespace Stockfish {
+
 static const size_t TH_STACK_SIZE = 8 * 1024 * 1024;
 
 template <class T, class P = std::pair<T*, void(T::*)()>>
@@ -57,10 +59,16 @@ public:
   void join() { pthread_join(thread, NULL); }
 };
 
+} // namespace Stockfish
+
 #else // Default case: use STL classes
 
+namespace Stockfish {
+
 typedef std::thread NativeThread;
 
+} // namespace Stockfish
+
 #endif
 
 #endif // #ifndef THREAD_WIN32_OSX_H_INCLUDED