kcms/users: Adjust for padding in overlay sheet

This commit fixes the right padding of the avatar sheet's grid view,
which otherwise can be overlapped by the scrollbar.

The commit is a backport for Plasma 5.24 of
!1468 (0c8abc18)
Which was squash-merged when its merge request was merged for Plasma
5.25. However this fix should be self-contained enough to backport to
the 5.24 LTS release.

BUG: 451031
FIXED-IN: 5.24.4