package/rsyslog: fix wrong symlink for systemd service

The symlink to /etc/systemd/system/syslog.service was pointing 4 levels up
to /usr/lib instead of 3 levels up.

This wasn't an issue on the actual system (/../ equals to / ) but was causing a
deadlink in the TARGET directory.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>