Replace sprintf with QString::asprintf

Summary: sprintf is marked as obsolete (https://doc.qt.io/qt-5/qstring-obsolete.html#sprintf) and causes a build fail against current Qt dev. Use asprintf instead

Test Plan: Ran ctest. impp test is failing, but it did before as well

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: winterz, vkrause, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21158