libspeexdec: Drop const qualifier to silence compiler warning.

libavcodec/libspeexdec.c:108: warning: passing argument 2 of ‘speex_bits_read_from’ discards qualifiers from pointer target type
/usr/include/speex/speex_bits.h:80: note: expected ‘char *’ but argument is of type ‘const uint8_t *’

libavcodec/libspeexdec.c

@@ -96,7 +96,7 @@ static int libspeex_decode_frame(AVCodecContext *avctx,
                                  void *data, int *data_size,
                                  AVPacket *avpkt)
 {
-    const uint8_t *buf = avpkt->data;
+    uint8_t *buf = avpkt->data;
     int buf_size = avpkt->size;
     LibSpeexContext *s = avctx->priv_data;
     int16_t *output = data, *end;