dm stripe: silence a couple sparse warnings

Eliminate the following sparse warnings:
drivers/md/dm-stripe.c:443:12: warning: symbol 'dm_stripe_init' was not declared. Should it be static?
drivers/md/dm-stripe.c:456:6: warning: symbol 'dm_stripe_exit' was not declared. Should it be static?

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-stripe.c

@@ -4,6 +4,7 @@
  * This file is released under the GPL.
  */
 
+#include "dm.h"
 #include <linux/device-mapper.h>
 
 #include <linux/module.h>