ipv6: Fix build failure when CONFIG_INET disabled

output_core.c, added in 3.2.66, is only needed and can only be
compiled when CONFIG_INET is enabled.

The condition in the Makefile is already correct upstream.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv6/Makefile

@@ -37,6 +37,7 @@ obj-$(CONFIG_NETFILTER)	+= netfilter/
 obj-$(CONFIG_IPV6_SIT) += sit.o
 obj-$(CONFIG_IPV6_TUNNEL) += ip6_tunnel.o
 
-obj-y += addrconf_core.o exthdrs_core.o output_core.o
+obj-y += addrconf_core.o exthdrs_core.o
+obj-$(CONFIG_INET) += output_core.o
 
 obj-$(subst m,y,$(CONFIG_IPV6)) += inet6_hashtables.o