[IPV6]: Remove unneeded pointer iph from ipcomp6_input() in net/ipv6/ipcomp6.c

This trivial patch removes the unneeded pointer iph, which is never used.

Signed-off-by: Micah Gruber <micah.gruber@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>