ipsec-tools: set SYSTEMD_SERVICE correctly to avoid building warning.

ipsec-tools-0.8.2: ipsec-tools: Files/directories were installed but not shipped
   /lib
   /lib/systemd
   /lib/systemd/system
   /lib/systemd/system/racoon.service [installed-vs-shipped]

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>