docker-engine: fix journald logging driver

The Docker engine can optionally log to systemd-journald. For this
driver to work correctly, Docker needs to build against
systemd-journald's client library.

This patch conditionally adds a build-time dependency on systemd and
enables compiling the journald driver in docker-engine if systemd is
used as the Buildroot init process.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>