avcodec/xiph: mark returned header pointers const from avpriv_split_xiph_headers()

Reviewed-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vorbis_parser.c

@@ -184,7 +184,7 @@ bad_header:
 static int vorbis_parse_init(AVVorbisParseContext *s,
                              const uint8_t *extradata, int extradata_size)
 {
-    uint8_t *header_start[3];
+    const uint8_t *header_start[3];
     int header_len[3];
     int ret;