matroska: don't complain about unknown elements

Matroska is an extensible format - unknown elements must be expected. It
shouldn't complain about such elements to the user either; it'll just
generate noise. The "error_recognition & AV_EF_EXPLODE" is completely,
wrong why would it explode on valid files?

It's still useful for debugging, so the message is left in place with a
higher log level.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
