dm raid: tidy includes

A dm target only needs to use include/linux dm headers.
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

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"
 
 /*