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