zebra: make declaration const in rtm_flag_dump()

* kernel_socket.c: (rtm_flag_dump) Use a const message pointer to be
in sync with commit ce0db9cb11c0bc2e7f89a7d042e50afa495556b4.
diff --git a/zebra/kernel_socket.c b/zebra/kernel_socket.c
index 8beac71..e8eaf28 100644
--- a/zebra/kernel_socket.c
+++ b/zebra/kernel_socket.c
@@ -236,7 +236,7 @@
 static void
 rtm_flag_dump (int flag)
 {
-  struct message *mes;
+  const struct message *mes;
   static char buf[BUFSIZ];
 
   buf[0] = '\0';