audio/bambootracker: Mark broken

The port is incompatible with rtaudio 6.x. The port broke after the
recent audio/rtaudio update.

The issue is known upstream and is being tracked under:

https://github.com/BambooTracker/BambooTracker/issues/492

Mark port broken while either an upstream fix or workaround is found.

Notified by: pkg-fallout