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.cpp

@@ -34,6 +34,8 @@
 
 using namespace std;
 
+namespace Stockfish {
+
 extern vector<string> setup_bench(const Position&, istream&);
 
 namespace {
@@ -369,3 +371,5 @@ Move UCI::to_move(const Position& pos, string& str) {
 
   return MOVE_NONE;
 }
+
+} // namespace Stockfish