Disabling MIDI compilation failure

"Jack L." <[email protected]> Sun, 18 Apr 2021 21:04:23 -0700
Newsgroups gmane.comp.audio.audacity.devel
Message-ID <CALeGphx9Hur7jnEXfGfs=ah+LP8MvQdbruzHhQGwjWFtSQKNbg@mail.gmail.com>
It looks like building audacity without MIDI support is causing a
compilation failure.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255186