Use the new KSysGuard namespace for KSysGuard related targets

Summary:
The targets in the KF5 namespace have been deprecated. This updates the
cmake files to use the new targets.

Test Plan: Builds correctly with most recent libksysguard.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29630