sddm-kcm: don't hardcode session file location.
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.
Reviewed By: Plasma, davidedmundson
Differential Revision: https://phabricator.kde.org/D16339