modules: don't break modules_install on external modules with no key.

The script still spits out an error ("Can't read private key") but we
don't break modules_install.

Reported-by: Bruno Wolff III <bruno@wolff.to>
Original-patch-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>