[build] do not complain about debian when DNS64 is not enabled (#1567)
[build] do not complain about debian when DNS64 is not enabled (#1567)

When building on Debian with NAT64=1 and DNS64=0, the setup script
will complain "dns64 is not tested on debian"

Since DNS64 is not necessary for NAT64, the script should continue in
this case instead of die.
openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix