Fix handling of Sunday in POSIX time-zone rules

This bug was causing the wrong time to be displayed in lxqt-panel under certain conditions. I could only reproduce when using the new upstream default of 'slim' for tzdata TZif files. I have organized all relevant information to reproduce and understand why it's rare in the below post on the lxqt github.

https://github.com/lxqt/lxqt/discussions/2107#discussioncomment-1630059
https://bugreports.qt.io/browse/QTBUG-90553
https://github.com/qt/qtbase/commit/38a72af4fd311219779f89b2b78cd56d879f61db