Use KSysGuard namespace for KSysGuard library targets
Summary:
Instead of using KF5 which is wrong since KSysGuard isn't part of
frameworks.
This needs a follow up patch to the ksysguard repo to make it use the new
namespace.
For backward compatibility, a KF5SysGuardConfig file is still installed that
aliases the new targets, but mentions they are deprecated.
Test Plan: Still builds
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29302