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/tune.h

@@ -24,6 +24,8 @@
 #include <type_traits>
 #include <vector>
 
+namespace Stockfish {
+
 typedef std::pair<int, int> Range; // Option's min-max values
 typedef Range (RangeFun) (int);
 
@@ -190,4 +192,6 @@ public:
 #define TUNE_CONDITIONS() int UNIQUE(c, __LINE__) = (Conditions.init(__COUNTER__), 0); \
                           TUNE(Conditions, set_conditions)
 
+} // namespace Stockfish
+
 #endif // #ifndef TUNE_H_INCLUDED