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/ucioption.cpp

@@ -31,6 +31,8 @@
 
 using std::string;
 
+namespace Stockfish {
+
 UCI::OptionsMap Options; // Global object
 
 namespace UCI {
@@ -190,3 +192,5 @@ Option& Option::operator=(const string& v) {
 }
 
 } // namespace UCI
+
+} // namespace Stockfish