Add AppStream metadata

Summary: Add AppStream Metadata for KSysGuard, since it's currently lacking.

Test Plan: Compile and install; the AppStream file is installed to /usr/share/metainfo/.

Reviewers: Plasma, apol, mak

Reviewed By: apol, mak

Subscribers: mak, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13772