tun: add __exit annotations to module exit func tun_cleanup()

Add missing __exit annotations to module exit func tun_cleanup().
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230814083000.3893589-1-william.xuanziyang@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/tun.c

@@ -3743,7 +3743,7 @@ static int __init tun_init(void)
 	return ret;
 }
 
-static void tun_cleanup(void)
+static void __exit tun_cleanup(void)
 {
 	misc_deregister(&tun_miscdev);
 	rtnl_link_unregister(&tun_link_ops);