avconv: Correct Copyright statement.

this more than 4000 line file has less than 600 lines that are not in ffmpeg.c
and they are largely cosmetic changes from ffmpeg.c.
Thus the copyright statement from ffmpeg.c should be used.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

avconv.c

 /*
- * avconv main
- * Copyright (c) 2000-2011 The libav developers.
+ * ffmpeg main
+ * Copyright (c) 2000-2003 Fabrice Bellard
  *
  * This file is part of FFmpeg.
  *