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)

(cherry picked from commit 41a66587) (edited)

edited:
- the -O2 patch is only applied for iOS on this branch

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>