lib32-rsyslog: fix wrong systemd server file

When enable multilib on x86-64, it fails to build lib32-rsyslog with
error:

| ERROR: Function failed: SYSTEMD_SERVICE_lib32-rsyslog value lib32-rsyslog.service does not exist

It uses wrong value ${PN}.service to set SYSTEMD_SERVICE. Correct it by
using BPN.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>