[Raring] Fix FTBS of dm-raid45 and re-enable it

[Stefan Bader]

This module is still a requirement for the dmraid package which
supplies support for various fakeRAIDs (software RAID with BIOS
support).
While we have now support for two on-disk meta-data formats with
mdadm (Intel Matrix Storage Manager (MSM) and Disk Data Format
(DDF)), there are likely still formats around that only dmraid
does support. Thus dropping that would loose functionality.
And even for IMSM and DDF migration needs to be carefully handled
from user-space (installer/startup).

-Stefan