kstyle: Fix class name to fix indeterminate progress bars animations

The Breeze implementation is older than qqc2-desktop-style framework,
and historically it used to use different base class. Times has
changed, along with base classes; but Breeze code did not -- instead
the problem was workarounded on a org.kde.desktop side first with a
dummy Timer that called updateItem(), then through more sophisticated
QtQuick Animations types (which delivered smoother results thanks to
being bound to refresh rate of a screen).

While this patch fixes the problem, it seems like a better solution
should be possible or more cleanup opportunities are available:

1. no other breeze animations invoke methods through QMetaObject system;
2. why and how an object would be something other than KQuickStyleItem?

(cherry picked from commit 8d88a824)