config.cc: Fix enable_ipv6_dns_lookup actual param name
config.cc: Fix enable_ipv6_dns_lookup actual param name

When adding option (and iterating through config refactoring)
the member name and the config param name got out of sync

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla