rtpdec: Don't pass non-const pointers to fmtp attribute parsing functions

This makes it clear that the individual parsing functions can't
touch the parsed out value.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec_mpeg4.c

@@ -274,7 +274,7 @@ static int aac_parse_packet(AVFormatContext *ctx, PayloadContext *data,
 
 static int parse_fmtp(AVFormatContext *s,
                       AVStream *stream, PayloadContext *data,
-                      char *attr, char *value)
+                      const char *attr, const char *value)
 {
     AVCodecContext *codec = stream->codec;
     int res, i;