dm init: fix const confusion for dm_allowed_targets array

A non const pointer to const cannot be marked initconst.
Mark the array actually const.

Fixes: 6bbc923d dm: add support to directly boot to a mapped device
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>