net: make struct tun_struct private to tun.c

There's no reason for this to be in the header, and it just hurts
recompile time.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Max Krasnyanskiy <maxk@qualcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>