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