liblogging: enable systemd compatibility libs

If systemd is chosen as init system, liblogging will enable the
installation of compatibility libraries in systemd.

When liblogging will be updated to depend on libsystemd.so, this
selection will have to be removed.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>