Fix build when KHotKeysDBusInterface is disabled/missing.

Summary:
By testing the variable itself (rather than its contents) we avoid a failure
when KHotKeysDBusInterface is disabled or missing as KHotKeysDBusInterface_FOUND
will not be defined.

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Differential Revision: https://phabricator.kde.org/D1278