rrdtool: add systemd support and fix QA issue

When building rrdtool in a building directory where systemd is configured
to be the init system, we would meet the following error.

ERROR: QA Issue: rrdtool: Files/directories were installed but not shipped in any package:
   /lib
   /lib/systemd
   /lib/systemd/system
   /lib/systemd/system/rrdcached.socket
   /lib/systemd/system/rrdcached.service

Fix this problem by adding systemd support to the rrdtool recipe.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>