kio_help: Fix crash in QCoreApplication when accessing help://.

A recent commit d428fc8e removed old
custom crash handling code in favor of KCrash. But this actually adds a
dependency on there being a valid QCoreApplication object.

I took from the example of the similar kio_file to simply create the
QCoreApplication here.

BUG:399709

Differential Revision: https://phabricator.kde.org/D16189