Silence a bunch of warnings under MSVC /W4

Still some remain, but are really the silly ones.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/movepick.h

@@ -47,6 +47,8 @@ struct SearchStack;
 
 class MovePicker {
 
+  MovePicker& operator=(const MovePicker&); // Silence a warning under MSVC
+
 public:
 
   enum MovegenPhase {