Don't convert built-in types to strings for qDebug.

This was introduced by 7ed15da3 (Core: QDebug and comparison operator
support metatypes,, 2013-03-20).

Change-Id: Id89aaffeee8d519ca73f0b52b1ac8b9d233cb5f1
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>