systemd: Add squeekboard to `Wants:`
Once squeekboard becomes systemd activatable (squeekboard!447) we can do away with the odd /usr/bin/osk-wayland
and just have a Wants:
relationsship in /usr/lib/systemd/user/gnome-session@phosh.target.d/session.conf
Related: #566 (closed), !773 (closed)
/cc @zhaofengli