Declare dm_allowed_targets as static to avoid the warning:

drivers/md/dm-init.c:39:12: warning: symbol 'dm_allowed_targets' was
not declared. Should it be static?

when compiling with C=1.
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>