Fix wrong UTC offset reporting

This bug is what causes https://together.jolla.com/question/39236/bug-google-calendar-events-off-by-12-hours/ and was actually fixed upstream http://marc.info/?l=kde-core-devel&m=134973661628231&w=2

The best way to fix it would be to sync with upstream but this version of kcalcore seems really old and I'd really like to see this bug fixed on my Jolla so I'm doing this pull request.