[kde-frameworks/kinit] Add USE flag 'caps'

RR#117125 (https://git.reviewboard.kde.org/r/117125) added support for
capabilities in kdeinit5 which used a SUID bit until now.
Using capabilities reduces possible security issues drastically and
gives kdeinit5 only those higher evelated privileges it really needs
to improve kdeinit5's behaviour in OOM scenarios.

Package-Manager: portage-2.2.10