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>

net/Makefile

@@ -19,9 +19,7 @@ obj-$(CONFIG_NETFILTER)		+= netfilter/
 obj-$(CONFIG_INET)		+= ipv4/
 obj-$(CONFIG_XFRM)		+= xfrm/
 obj-$(CONFIG_UNIX)		+= unix/
-ifneq ($(CONFIG_IPV6),)
-obj-y				+= ipv6/
-endif
+obj-$(CONFIG_NET)		+= ipv6/
 obj-$(CONFIG_PACKET)		+= packet/
 obj-$(CONFIG_NET_KEY)		+= key/
 obj-$(CONFIG_BRIDGE)		+= bridge/