net: Enter net/ipv6/ even if CONFIG_IPV6=n

exthdrs_core.c and addrconf_core.c in net/ipv6/ contain bits which
must be made available even if IPv6 is disabled.

net/ipv6/Makefile already correctly includes them if CONFIG_IPV6=n
but net/Makefile prevents entering the subdirectory.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>