License maintenance: GPLv2 -> GPLv2+

Summary:
This should now cover all remaining GPLv2-only files in pimcommon,
excluding test code. This is based on existing approval from
https://techbase.kde.org/Projects/KDE_Relicensing.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16477