Prevent a leak of QXcbXSettingsPrivate

The private was not deleted. Adding the dtor in turn
causes a warning about not having a virtual dtor
in the base class, so add that as well.

Change-Id: I24a90caf2cf6192a6f17cf5af96b8f77010d9127
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Android: Fix more exceptions in ExtractStyle.java on Android L

Task-number: QTBUG-40120
Change-Id: I949f19ced4eefcf7a40e64ad63e316765c6f2f48
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>