dm unstripe: add "dm-unstriped" module alias

This target's kernel module being named dm-unstripe.ko doesn't allow
lvm2's DM module autoload capability to load the dm-unstripe.ko
because lvm2 looks for dm-unstriped.ko due to the target name being
"unstriped".

Add the "dm-unstriped" module alias to resolve this oversight.

NOTE: this isn't needed for the "striped" target, despite its source
file being named dm-stripe.c, because it is part of dm-mod.ko.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>