proto: permit icmp-in-ipv6 and icmpv6-in-ipv4

proto: permit icmp-in-ipv6 and icmpv6-in-ipv4

Jozsef points out that
 meta l4proto icmp icmp type destination-unreachable

is hard to read.  So, lets just add icmp/icmpv6 to
ip/ip6 protocol base so users can just go with

 icmp type destination-unreachable

and let nft fill in needed dependency.
After this patch, the recent patch to not remove the
dependency can be reverted again.

Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>