d-i: Fix module name for dm-raid45
Another doh! Probably just went unnoticed because not so
many people install into fakeraid or mdadm which became
more intelligent at some point.
In my testing it was not intelligent enough (or me). While
mdadm detected the softraid partitions/volumes, it would
not make then writeable.
Talking to cjwatson he asked me to add back the dm-raid45
module. So here the patch is.