audio/cava: Add pkg-message

[jbo]

Summary

Add pkg-message to inform new users about required setup in
order to visualization the played back audio with OSS or sndio.

PR: 277152
Approved by: Adam Jimerson <[e-mail]> (maintainer, timeout)

[tcberner]

looks sane enough

Not quite sure that an portrevision bump is going to serve anyone though -- those who already installed it won't see it, and those who don't don't profit from a new pkg.

strike that, those who don't profit