Fix background-image warning

Summary:
This fixes the following warning:

    Gtk-WARNING **: Theme parsing error: gtk.css:3139:22: Not a valid image

It was changed by commit 01a86601 but the change is not in D6688 (so
it looks like a copy-paste error while pushing the revision).

Reviewers: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7836