Don't notify ksplash in our wayland session

Summary:
KSplash doesn't expect a kwin notification on wayland as kwin is started
before ksplash.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: broulik, zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23770