libinput/connection: Don't hardcode seat0 anymore
Summary: use a new function to get the logind, instead of assuming seat0
Test Plan: kwin still starts
Reviewers: KWin, davidedmundson
Reviewed By: davidedmundson
Subscribers: davidedmundson, rkflx, graesslin, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D9552