systemd: Add squeekboard to `Wants:`
Once squeekboard becomes systemd activatable (https://source.puri.sm/Librem5/squeekboard/-/merge_requests/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: https://source.puri.sm/Librem5/phosh/-/issues/566, https://source.puri.sm/Librem5/phosh/-/merge_requests/773
/cc @zhaofengli