lavc/hwaccel: fix header copyright

It was done on a whim because of the FATE header check and was actually
meant to be removed before pushing.

Also, nobody in review spotted it.

Reviewed-by: wm4

libavcodec/hwaccel.h

 /*
- * This file is part of FFmpeg and was stolen from Libav.
+ * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public