Syslog application is not started when only some sinks are configured to use it.

Reported in https://groups.google.com/forum/#!topic/rabbitmq-users/1yk3akqVmYE

When trying to configure only some categories to use `syslog_lager_backend`, the `syslog` application does not start by default.
This requires starting manually every time rabbitmq node restarts, which is inconvenient.