avoid a 'discards qualifiers from pointer' warning

Originally committed as revision 8900 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegaudio_parser.c

@@ -186,7 +186,7 @@ static int mpegaudio_parse(AVCodecParserContext *s1,
         if(s->frame_size > 0 && buf_ptr - buf == s->inbuf_ptr - s->inbuf
            && buf_size + buf_ptr - buf >= s->frame_size){
             if(s->header_count > 0){
-                *poutbuf = buf;
+                *poutbuf = (uint8_t *)buf;
                 *poutbuf_size = s->frame_size;
             }
             buf_ptr = buf + s->frame_size;