Specify category for GHNS

Summary:
With my last fixes in Synchrotron, the phenomenon of empty category
names is gone and no content is assigned to an empty category name
anymore. With this patch, GHNS again finds the QtHelp files.

Test Plan: Manual testing.

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2064