rtmp: Add credit/copyright to librtmp authors for parts of the RTMPE code

Our implementation of RTMPE is heavily based on librtmp.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtmpcrypt.c

 /*
  * RTMPE network protocol
+ * Copyright (c) 2008-2009 Andrej Stepanchuk
+ * Copyright (c) 2009-2010 Howard Chu
  * Copyright (c) 2012 Samuel Pitoiset
  *
  * This file is part of Libav.

libavformat/rtmpdh.c

 /*
  * RTMP Diffie-Hellmann utilities
+ * Copyright (c) 2009 Andrej Stepanchuk
+ * Copyright (c) 2009-2010 Howard Chu
  * Copyright (c) 2012 Samuel Pitoiset
  *
  * This file is part of Libav.