nano: fix build breakage with libmagic

As reported by Cassiano Martin in bug #6692 if host == target the nano
package can pick up the host libmagic and break.
So add a check to see if the file package is enabled and use it,
otherwise just disable libmagic support.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>