tun: Fix code style issues in <linux/if_tun.h>

This fixes the following code style problem:
- WARNING: please, no spaces at the start of a line
- CHECK: Please use a blank line after
         function/struct/union/enum declarations

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>