uapi: fix linux/ip6_tunnel.h userspace compilation errors

Include <linux/if.h> and <linux/in6.h> to fix the following
linux/ip6_tunnel.h userspace compilation errors:

/usr/include/linux/ip6_tunnel.h:23:12: error: 'IFNAMSIZ' undeclared here (not in a function)
  char name[IFNAMSIZ]; /* name of tunnel device */
/usr/include/linux/ip6_tunnel.h:30:18: error: field 'laddr' has incomplete type
  struct in6_addr laddr; /* local tunnel end-point address */

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>