netfilter: x_tables: add module alias for icmp matches

The icmp matches are implemented in ip_tables and ip6_tables,
respectively, so for normal iptables they are always available:
those modules are loaded once iptables calls getsockopt() to fetch
available module revisions.

In iptables-over-nftables case probing occurs via nfnetlink, so
these modules might not be loaded.  Add aliases so modprobe can load
these when icmp/icmp6 is requested.

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