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/movepick.cpp

@@ -20,6 +20,8 @@
 
 #include "movepick.h"
 
+namespace Stockfish {
+
 namespace {
 
   enum Stages {
@@ -263,3 +265,5 @@ top:
   assert(false);
   return MOVE_NONE; // Silence warning
 }
+
+} // namespace Stockfish