dm raid: tidy includes

A dm target only needs to use include/linux dm headers.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
index efa960f..d4e95b2 100644
--- a/drivers/md/dm-raid.c
+++ b/drivers/md/dm-raid.c
@@ -9,9 +9,10 @@
 
 #include "md.h"
 #include "raid5.h"
-#include "dm.h"
 #include "bitmap.h"
 
+#include <linux/device-mapper.h>
+
 #define DM_MSG_PREFIX "raid"
 
 /*