Correctly implementg selDepth feature

Send to GUI the deepest search depth apart from
qsearch of the PV line.

No functional change.

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

src/thread.h

@@ -68,6 +68,7 @@ enum ThreadState
 };
 
 struct Thread {
+  int maxPly;
   Lock sleepLock;
   WaitCondition sleepCond;
   volatile ThreadState state;