matroskadec: Remove redundant const

The typedef used to define EbmlSyntax already includes a const qualifier
so that it is unnecessary to include another const qualifier in future
definitions and declarations. Given that MSVC warns about this, this
commit removes these redundant const qualifiers.
Suggested-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>