Rename library/target to KF5::DAV

Summary: Prepares this for the move to Frameworks after 20.04.

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Subscribers: mlaurent, cgiboudeaux, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26762