QtHelp CSS images fix

Summary:
Fix broken CSS images for QtHelp pages.

Old version:

New version:

Test Plan: Tested on master branch with Qt 5.5.1 and 5.7.0.

Reviewers: kfunk, KDevelop

Reviewed By: kfunk, KDevelop

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3713