Replace sprintf with QString::asprintf

Summary: sprintf is obsolete and causes build fail with current Qt dev. Use asprintf

Test Plan:
ctest

Some tests fail, but they did before:
472 - Compat-libical3-AppleICal_1.5.ics (Failed)
473 - Compat-libical3-Evolution_2.8.2_timezone_test.ics (Failed)
475 - Compat-libical3-KOrganizer_3.1a.ics (Failed)
477 - Compat-libical3-MSExchange.ics (Failed)
478 - Compat-libical3-Mozilla_1.0.ics (Failed)

Reviewers: KDE PIM, vkrause

Reviewed By: KDE PIM, vkrause

Subscribers: winterz, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21160