net: sockets: Do not lookup DNS AAAA entries if IPv6 is disabled

Do not lookup DNS AAAA entries if IPv6 is disabled. The result can not
be used anyway because "struct sockaddr" does not have enough space for
IPv6 address in that case.

Signed-off-by: Stig Bjørlykke <stig.bjorlykke@nordicsemi.no>