xfrm: Mark flowi arg to ->fill_dst() const.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 2c0927b..c77407f 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -279,7 +279,7 @@
 					     int nfheader_len);
 	int			(*fill_dst)(struct xfrm_dst *xdst,
 					    struct net_device *dev,
-					    struct flowi *fl);
+					    const struct flowi *fl);
 };
 
 extern int xfrm_policy_register_afinfo(struct xfrm_policy_afinfo *afinfo);