Set XDG_SESSION_TYPE=wayland in startplasmacompositor

Summary:
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

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18233