EbmlSInteger: use INT64_C instead of EBML_PRETTYLONGINT
EbmlSInteger: use INT64_C instead of EBML_PRETTYLONGINT

This is the standard way to handle it, it works with more than GCC compatible
compilers.

See https://en.cppreference.com/w/cpp/types/integer

Replaces #59

Matroska-Org/libebml,Matroska-Org/libebml