lang/gcc6-aux: unbreak fetch after 620968a43a5f

> ada-bootstrap.amd64.freebsd.120.tar.bz2 is not in /usr/ports/lang/gcc6-aux/distinfo.

> Either /usr/ports/lang/gcc6-aux/distinfo is out of date, or

> ada-bootstrap.amd64.freebsd.120.tar.bz2 is spelled incorrectly.

• Error code 1

Reported by: pkg-fallout