login-sessions: (Correctly) set the correct filename depending on default session type

For distributions aiming to ship the Wayland session by default, allow
the Wayland session file to be installed as "plasma.desktop". Otherwise,
revert to status quo where the X11 session is installed as "plasma.desktop".

This time around, ensure that CMake stanza is correctly filled out.

This fixes commit b93a0639.

This reverts commit a6b28911.