Add NLMSG_TAIL macro pointing to the tail of a netlink message

(Logical change 1.127)
diff --git a/include/libnetlink.h b/include/libnetlink.h
index 08be752..40587a0 100644
--- a/include/libnetlink.h
+++ b/include/libnetlink.h
@@ -46,5 +46,8 @@
 extern int rtnl_from_file(FILE *, rtnl_filter_t handler,
 		       void *jarg);
 
+#define NLMSG_TAIL(nmsg) \
+	((struct rtattr *) (((void *) (nmsg)) + NLMSG_ALIGN((nmsg)->nlmsg_len)))
+
 #endif /* __LIBNETLINK_H__ */