[systemd] Move kwin to "wants"

As kwin-wayland is still spawned manually by us for Plasma 5.21.


(cherry picked from commit 07b2d67f)

startkde/systemd/plasma-workspace@.target

 [Unit]
 Description=KDE Plasma Workspace
-Requires=plasma-kwin_%i.service plasma-core.target xdg-desktop-autostart.target graphical-session.target
-Wants=plasma-restoresession.service plasma-xembedsniproxy.service plasma-gmenudbusmenuproxy.service plasma-powerdevil.service plasma-ksplash-ready.service plasma-polkit-agent.service plasma-baloo.service plasma-foreground-booster.service
+Requires=plasma-core.target xdg-desktop-autostart.target graphical-session.target
+Wants=plasma-kwin_%i.service plasma-restoresession.service plasma-xembedsniproxy.service plasma-gmenudbusmenuproxy.service plasma-powerdevil.service plasma-ksplash-ready.service plasma-polkit-agent.service plasma-baloo.service plasma-foreground-booster.service
 BindsTo=graphical-session.target
 Before=graphical-session.target xdg-desktop-autostart.target plasma-ksplash-ready.service plasma-restoresession.service