xfrm: interface: use IS_REACHABLE to avoid some compile errors

kernel test robot reported some compile errors:

  ia64-linux-ld: net/xfrm/xfrm_interface.o: in function `xfrmi4_fini':
  net/xfrm/xfrm_interface.c:900: undefined reference to `xfrm4_tunnel_deregister'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:901: undefined reference to `xfrm4_tunnel_deregister'
  ia64-linux-ld: net/xfrm/xfrm_interface.o: in function `xfrmi4_init':
  net/xfrm/xfrm_interface.c:873: undefined reference to `xfrm4_tunnel_register'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:876: undefined reference to `xfrm4_tunnel_register'
  ia64-linux-ld: net/xfrm/xfrm_interface.c:885: undefined reference to `xfrm4_tunnel_deregister'

This happened when set CONFIG_XFRM_INTERFACE=y and CONFIG_INET_TUNNEL=m.
We don't really want xfrm_interface to depend inet_tunnel completely,
but only to disable the tunnel code when inet_tunnel is not seen.

So instead of adding "select INET_TUNNEL" for XFRM_INTERFACE, this patch
is only to change to IS_REACHABLE to avoid these compile error.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: da9bbf05

 ("xfrm: interface: support IPIP and IPIP6 tunnels processing with .cb_handler")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>