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/main.cpp

@@ -28,6 +28,8 @@
 #include "tt.h"
 #include "uci.h"
 
+using namespace Stockfish;
+
 int main(int argc, char* argv[]) {
 
   std::cout << engine_info() << std::endl;