Cocoa: Fix memory leak in the accessibility code.

QCocoaAccessibleElement takes ownership of the
QAccessibleinterface pointer. Delete it in dealloc().

Change-Id: I45a5540b9cf564c639bfa119ff4882008d63fd96
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>

src/plugins/platforms/cocoa/qcocoaaccessibilityelement.mm

@@ -73,6 +73,7 @@ static QAccessibleInterface *acast(void *ptr)
 
 - (void)dealloc {
     [super dealloc];
+    delete acast(accessibleInterface);
 }
 
 - (BOOL)isEqual:(id)object {