net: ipv6: Fix RA RDNSS and DNS_RESOLVER interdependencies

A support for RDNSS option should not force DNS subsystem being enabled
in the system, especially that the option is enabled by default. It
should work the other way around - the option is supported only if DNS
was enabled by the user.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>