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/movegen.cpp

@@ -21,6 +21,8 @@
 #include "movegen.h"
 #include "position.h"
 
+namespace Stockfish {
+
 namespace {
 
   template<GenType Type, Direction D>
@@ -362,3 +364,5 @@ ExtMove* generate<LEGAL>(const Position& pos, ExtMove* moveList) {
 
   return moveList;
 }
+
+} // namespace Stockfish