airtun-ng and airodump-ng _do_ build on 6.x, so build them by default.
Also added a new pkg-message to explain the reason that airmon-ng is missing.

Thanks for the advises to:
Eric P. Scott, Jan Henrik Sylvester and zion.

PR: 112189
Submitted by: Lars Engels <lars.engels@0x20.net> (maintainer)