libhb: set audio description in LookForAudio (scan.c).

All audio tracks go through it since SVN revision 5424.

This is where we have the most information about each audio track.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5996 b64f7644-9d1e-0410-96f1-a4d463321fa5