netxen: include ipv6.h (fixes build failure)

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: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netxen/netxen_nic_main.c

@@ -41,6 +41,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/if_vlan.h>
 #include <net/ip.h>
+#include <linux/ipv6.h>
 
 MODULE_DESCRIPTION("NetXen Multi port (1/10) Gigabit Network Driver");
 MODULE_LICENSE("GPL");