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