kcm/kwinrules: Fix header in properties list
This provides a better fix for the header positioning bug in the properties list sheet, instead of the previous workaround (which wasn't enough in some situations)
Upstream bugfix (in the Kirigami component) is not advisable as it could cause unintended consequencies in other uses/apps
See also: https://bugs.kde.org/show_bug.cgi?id=422289
BUG: 421583 FIXED-IN: 5.24