Fix annoying warning

Summary:
Warning for every plugin was:
The file includes the moc file "generator_tiff.moc", but does not contain a Q_OBJECT, Q_GADGET, Q_NAMESPACE, K_PLUGIN_FACTORY_WITH_JSON, K_PLUGIN_FACTORY, K_PLUGIN_FACTORY_WITH_JSON or K_PLUGIN_FACTORY macro.

Test Plan: Built

Reviewers: aacid

Reviewed By: aacid

Subscribers: #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D10022