matroskadec: Mark variable as av_unused.

This avoids unused variable warnings when zlib/bzlib are not available.

libavformat/matroskadec.c

@@ -939,7 +939,7 @@ static int matroska_decode_buffer(uint8_t** buf, int* buf_size,
     uint8_t* data = *buf;
     int isize = *buf_size;
     uint8_t* pkt_data = NULL;
-    uint8_t* newpktdata;
+    uint8_t av_unused *newpktdata;
     int pkt_size = isize;
     int result = 0;
     int olen;