use correct filepath

Summary:
Seems like someone forgot to consistently apply a change. Maybe the path
construction should be factored into a reusable lambda?

Test Plan:
Open Okular, open settings > configure Okular. Without the patch, you'll get a
"QFSFileEngine::open: No file name specified" warning, with the patch it
disappears. Note that there is also an unrelated "Illegal icon group:  7"
warning from KIconThemes, but I'm 99% certain that this is on KIconThemes doing
a bogus check in KIconEffect::apply, not on Okular

Reviewers: #okular, hetzenecker, aacid

Reviewed By: #okular, aacid

Subscribers: aacid, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D12001