cleanup - remove tun.h

Longer term we should probably just merge the clat_config
and tun_data structs, but not just yet.

Test:
  git grep '(^|[^_])tun[.]h'
  comes up empty

Bug: 144730808
Test: atest clatd_test netd_integration_test
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I7c3a986584aa3d02519545fb928457b61247ed14
diff --git a/config.h b/config.h
index 0152df0..95cdefa 100644
--- a/config.h
+++ b/config.h
@@ -21,6 +21,14 @@
 #include <linux/if.h>
 #include <netinet/in.h>
 
+#include "ring.h"
+
+struct tun_data {
+  char device4[IFNAMSIZ];
+  int read_fd6, write_fd6, fd4;
+  struct packet_ring ring;
+};
+
 struct clat_config {
   struct in6_addr ipv6_local_subnet;
   struct in_addr ipv4_local_subnet;