tuntap: fix sparse warning

Make tun_enable_queue() static to fix the sparse warning:

drivers/net/tun.c:399:19: sparse: symbol 'tun_enable_queue' was not declared. Should it be static?
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -396,7 +396,7 @@ static void tun_disable_queue(struct tun_struct *tun, struct tun_file *tfile)
 	++tun->numdisabled;
 }
 
-struct tun_struct *tun_enable_queue(struct tun_file *tfile)
+static struct tun_struct *tun_enable_queue(struct tun_file *tfile)
 {
 	struct tun_struct *tun = tfile->detached;