mkv: fix statement has no effect warning

Loop revised to c11 style rather than just dropping the unnecessary `t;`
upon request by Alexandre in review.

```
../../modules/demux/mkv/matroska_segment_parse.cpp: In member function 'void mkv::matroska_segment_c::ParseTracks(libmatroska::KaxTracks*)':
WARNING : ../../modules/demux/mkv/matroska_segment_parse.cpp:1102: 10:  statement has no effect [-Wunused-value]
 1102 |     for (t; t != matroska_segment_c::tracks.end(); ++t)
      |          ^
```