lavf/rtmpcrypt: Add a cast to silence an unavoidable warning.

The av_rc4_crypt() documentation allows src == dst.

Silences the following warning:
libavformat/rtmpcrypt.c:304:36: warning: passing argument 2 of 'av_rc4_crypt' discards 'const' qualifier from pointer target type

Reported-by: Reino Wijnsma