rsyslog: Fix systemd service creation

The rsyslog.service is installed by rsyslog, but the link is not created
so the service is not enabled.
In order to install the service file, use the correct configure flag.

So the INSTALL_INIT_SYSTEMD will create the link, it will also create
another link which is due to the fact that the rsyslog.service contains
an Alias=

If we were to use systemctl enable to enable the service, it would
create both,so we mimic that.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>