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

libavformat/rtmpcrypt.c

@@ -301,7 +301,7 @@ static int rtmpe_write(URLContext *h, const uint8_t *buf, int size)
 
     if (rt->handshaked) {
         /* encrypt data to send to the server */
-        av_rc4_crypt(&rt->key_out, buf, buf, size, NULL, 1);
+        av_rc4_crypt(&rt->key_out, (uint8_t *)buf, buf, size, NULL, 1);
     }
 
     if ((ret = ffurl_write(rt->stream, buf, size)) < 0)