mdadm: Set allowedReferences to only include libc.

The updates we had in the past (7719f7f2 and 1faf6106

) broke the mdadm
installer test and systems in the wild using mdraid because newer mdadm
versions contained a self-referenc to its own store path.

Instead of putting a big warning about updating in the package
expression, let's just add allowedReferences so the build immediately
fails if there is a self-reference.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>