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>
diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index 9cdd12a..cedbbd8 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -86,6 +86,7 @@
 header-y += if_ppp.h
 header-y += if_slip.h
 header-y += if_strip.h
+header-y += if_tun.h
 header-y += if_tunnel.h
 header-y += in6.h
 header-y += in_route.h
@@ -229,7 +230,6 @@
 unifdef-y += if_pppol2tp.h
 unifdef-y += if_pppox.h
 unifdef-y += if_tr.h
-unifdef-y += if_tun.h
 unifdef-y += if_vlan.h
 unifdef-y += if_wanpipe.h
 unifdef-y += igmp.h