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

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 2de3dae..2c0927b 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -273,7 +273,7 @@
 	void			(*decode_session)(struct sk_buff *skb,
 						  struct flowi *fl,
 						  int reverse);
-	int			(*get_tos)(struct flowi *fl);
+	int			(*get_tos)(const struct flowi *fl);
 	int			(*init_path)(struct xfrm_dst *path,
 					     struct dst_entry *dst,
 					     int nfheader_len);