sddm-kcm: don't hardcode session file location.

Summary:
The sddm-kcm uses /usr/share/xsessions and /usr/share/wayland-sessions pathes, but in FreeBSD the correct path is /usr/local/share/*.

This cause, that in sddm page of systemsettings, in the part of automatic login, the sessions are empty.

See: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232356

Reviewers: #freebsd, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16339