Set XDG_SESSION_TYPE=wayland in startplasmacompositor
KWorkSpace::detectPlatform among others use that, so make sure it's set.
On 5.12 it has export QT_QPA_PLATFORM=wayland already, so it's not necessary (?).
Test Plan: I had that patch applied locally for basically ever, works fine.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Differential Revision: https://phabricator.kde.org/D18233