Really fix appdata location

Summary:
7db6c1f5081f65f55ee7da065148b1b15272e22c broke installation, because
SHARE_INSTALL_DIR is nowhere defined.
The previous DATA_INSTALL_DIR is defined as "share/apps" which indeed
isn't correct either. SHARE_INSTALL_*PREFIX* exists and points to
"${CMAKE_INSTALL_PREFIX}/share" by default.

Reviewers: KDE Games, rdieter, arojas

Differential Revision: https://phabricator.kde.org/D8259