[IPV6]: Fix build with fwmark disabled.

Based upon a patch by Brian Haley.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 649350b..d83844d 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -703,7 +703,9 @@
 			.ip6_u = {
 				.daddr = iph->daddr,
 				.saddr = iph->saddr,
+#ifdef CONFIG_IPV6_ROUTE_FWMARK
 				.fwmark = skb->nfmark,
+#endif
 				.flowlabel = (* (u32 *) iph)&IPV6_FLOWINFO_MASK,
 			},
 		},