Fix compile error in debug mode

Build broken by commit 3141490374
where we renamed move_is_ok() in is_ok() and this clashes
with the same named method in Position that overrides the
move's one causing compile errors.

The fix is to rename the method in Position.

No functional change.

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

src/position.h

@@ -203,7 +203,7 @@ public:
   void set_nodes_searched(int64_t n);
 
   // Position consistency check, for debugging
-  bool is_ok(int* failedStep = NULL) const;
+  bool pos_is_ok(int* failedStep = NULL) const;
   void flip_me();
 
   // Global initialization