Add audio/libltc

Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode
data as a Manchester-Biphase encoded audio signal. The audio signal is
commonly recorded on a VTR track or other storage media.

libltc provides functionality to encode and decode LTC audio from/to SMPTE
or EBU timecode, including SMPTE date support.

(nia)