Fix Clang compile error due to missing cast

crypto/conf/conf_def.c:148:18: error: assigning to 'void *' from 'const
unsigned short [256]' discards qualifiers
      [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
        conf->meth_data = CONF_type_default;
                        ^ ~~~~~~~~~~~~~~~~~
1 error generated.
make: *** [crypto/conf/conf_def.o] Error 1