contrib: matroska: update libmatroska to 1.6.0

No need to force fvisibility=hidden explicitly anymore.

Log why the -O2 is needed (this still feels like an odd fix)
15 jobs for !22 with tst-contribs in 46 minutes and 27 seconds (queued for 1 second)
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uwp64-libvlc-llvm

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