dm init: Set file local variable static

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>

drivers/md/dm-init.c

@@ -36,7 +36,7 @@ struct dm_device {
 	struct list_head list;
 };
 
-const char * const dm_allowed_targets[] __initconst = {
+static const char * const dm_allowed_targets[] __initconst = {
 	"crypt",
 	"delay",
 	"linear",