Fix a MSVC warning at W4

Warning is C4512 (assignment operator could not be generated)

Now, apart the foreign syzygy code, everything compiles
without warnings at warning level 4.

No functional change.

src/endgame.h

@@ -90,7 +90,7 @@ struct Endgame : public EndgameBase<T> {
   T operator()(const Position&) const;
 
 private:
-  const Color strongSide, weakSide;
+  Color strongSide, weakSide;
 };