Pub/sub examples in ign-transport: why don't I see `/foo` in the topic list?

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Hello,

If I do the following:

• Build the examples in https://github.com/ignitionrobotics/ign-transport, default branch (ign-transport10).
• Run examples/publisher.cc in a console.
• Run examples/subscriber.cc in another console.

Both programs appear to work, with the subscriber indicating that it's receiving messages from the publisher. However, if I then do ign topic -l, I don't get anything back. I would expect to see /foo listed. What am I missing?