Move game_phase() to Position

It seems a more natural to place this
function there.

No functional change.

src/position.h

@@ -156,6 +156,7 @@ public:
 
   // Other properties of the position
   Color side_to_move() const;
+  Phase game_phase() const;
   int game_ply() const;
   bool is_chess960() const;
   Thread* this_thread() const;