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/uci.h

@@ -24,6 +24,8 @@
 
 #include "types.h"
 
+namespace Stockfish {
+
 class Position;
 
 namespace UCI {
@@ -78,4 +80,6 @@ Move to_move(const Position& pos, std::string& str);
 
 extern UCI::OptionsMap Options;
 
+} // namespace Stockfish
+
 #endif // #ifndef UCI_H_INCLUDED