netfilter: xt_ipcomp: add "ip[6]t_ipcomp" module alias name

Otherwise, user cannot add related rules if xt_ipcomp.ko is not loaded:
  # iptables -A OUTPUT -p 108 -m ipcomp --ipcompspi 1
  iptables: No chain/target/match by that name.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>