net: ipv6: Fix Kconfig dependencies when native IPv6 is disabled

In case native IPv6 is disabled, Kconfig entries related to native IPv6
stack should not be enabled. Otherwise, circular dependencies can be
created if native stack is disabled, as in case of recent changes in
NET_IPV6_RA_RDNSS option (where a dependency for a native stack module
was enabled).

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