GRMLBASE/39-modprobe: do not expect all files in /etc/modprobe.d to be used

GRMLBASE/39-modprobe: do not expect all files in /etc/modprobe.d to be used

Quotin from fcopy(8) about its -i option:

| Ignore warnings about no matching class and non-existing source directories. These warnings will not set the exit code to 1.

By adding this, we can support configurations of users
who have additional files in their /etc/modprobe.d setup,
though don't want to necessarily always install them.