sysstat: Use sysstat.service in source for cron with systemd

sysstat: Use sysstat.service in source for cron with systemd

The sysstat.service script in source launchs sysstat-collect and sysstat-summary
services when cron is installed with systemd. At this time, the upstream
sysstat.service must be installed.

Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>