rtpdec_xiph: fix function return type

parse_packed_headers() returns either zero or a negative error code
so its return type must be signed.
Signed-off-by: NMans Rullgard <mans@mansr.com>

libavformat/rtpdec_xiph.c

@@ -243,7 +243,7 @@ static int get_base128(const uint8_t ** buf, const uint8_t * buf_end)
 /**
  * Based off parse_packed_headers in Vorbis RTP
  */
-static unsigned int
+static int
 parse_packed_headers(const uint8_t * packed_headers,
                      const uint8_t * packed_headers_end,
                      AVCodecContext * codec, PayloadContext * xiph_data)