libcgroup_3.1.0: fix build on non-systemd systems

backport upstream commit 592dcdcf243576bd2517d3da9bc18990de08e37e
to fix packaging when building with --enable-systemd=no

Signed-off-by: Adriaan Schmidt <adriaan.schmidt@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>