matroskademux: improve index memory usage

Re-arrange order of index entry struct members to avoid padding
bytes in the middle of the struct, thus potentially reducing the
overall size of the struct and reducing memory used by the index.
On Linux x86_64 the size goes down from 32 bytes to 24 bytes for
each index entry.

gst/matroska/matroska-ids.h

@@ -616,9 +616,9 @@ typedef struct _GstMatroskaTrackSubtitleContext {
 
 typedef struct _GstMatroskaIndex {
   guint64        pos;      /* of the corresponding *cluster*! */
-  guint16        track;    /* reference to 'num' */
   GstClockTime   time;     /* in nanoseconds */
   guint32        block;    /* number of the block in the cluster */
+  guint16        track;    /* reference to 'num' */
 } GstMatroskaIndex;
 
 typedef struct _Wavpack4Header {