Exclude the kwin process when checking if we are in wayland

Kwin does not speak wayland to itself. Recent changes make use of
QWaylandClientExtension that understandably assumes the wayland QPA is
in use.

Kwin's QPA name "wayland-org.kde.kwin.qpa" matches the check
KWindowSystem::isPlatformWayland() which internally does
platformName.startsWith("wayland")

We have no reason to sync application colour schemes or appmenu bars to
kwin from inside the QPT, so we can just tighten our check for which QPA
is in use.