include/dm.h: fix inclusion guard

cc: Simon Glass <sjg@chromium.org>
Signed-off-by: NJeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: NSimon Glass <sjg@chromium.org>

include/dm.h

@@ -5,7 +5,7 @@
  */
 
 #ifndef _DM_H_
-#define _DM_H
+#define _DM_H_
 
 #include <dm/device.h>
 #include <dm/platdata.h>