fix: static class member MAX_DATA_ENTRIES is not defined -> linker error
fix: static class member MAX_DATA_ENTRIES is not defined -> linker error

Static class members usually have to be defined explicitly which plays not
well with header only libraries. Moving the constant into the ELFIO scope
works around that problem.

The problem only shows up at -O0. Higher optimisations levels manage to
completely remove the references to the static member.

niosHD/ELFIO,niosHD/ELFIO,serge1/ELFIO,niosHD/ELFIO,niosHD/ELFIO,serge1/ELFIO,serge1/ELFIO