Fixes a build error in absence of CONFIG_IPV6:

drivers/net/netxen/netxen_nic_main.c:1189: error: implicit declaration of function 'ipv6_hdr'
drivers/net/netxen/netxen_nic_main.c:1189: error: invalid type argument of '->'
Reported-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>