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/thread.cpp

@@ -253,7 +253,7 @@ template <bool Fake>
 Value ThreadsManager::split(Position& pos, SearchStack* ss, Value alpha, Value beta,
                             Value bestValue, Depth depth, Move threatMove,
                             int moveCount, MovePicker* mp, int nodeType) {
-  assert(pos.is_ok());
+  assert(pos.pos_is_ok());
   assert(bestValue >= -VALUE_INFINITE);
   assert(bestValue <= alpha);
   assert(alpha < beta);