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/misc.h

@@ -28,6 +28,8 @@
 
 #include "types.h"
 
+namespace Stockfish {
+
 std::string engine_info(bool to_uci = false);
 std::string compiler_info();
 void prefetch(void* addr);
@@ -143,4 +145,6 @@ namespace CommandLine {
   extern std::string workingDirectory; // path of the working directory
 }
 
+} // namespace Stockfish
+
 #endif // #ifndef MISC_H_INCLUDED