include/uapi/linux/if_tunnel.h: include linux/if.h, linux/ip.h and linux/in6.h

Fixes userspace compilation errors like:

error: field ‘iph’ has incomplete type
error: field ‘prefix’ has incomplete type

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>