Fix build with Qt 5.9

Summary:
Otherwise we get the following error:
/home/apol/devel/frameworks/khtml/src/khtml_factory.h:31: Error: Plugin Metadata file "" could not be opened: file to open is a directory

Arguably it's something to fix upstream, but also having `FILE ""` doesn't add
much either.

BUG: 377490

Test Plan: Now it compiles, tests pass, khelpcenter runs.

Reviewers: #frameworks, kfunk

Reviewed By: kfunk

Subscribers: mpyne, ltoscano, #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D5392