dm: Fix typo in include-guard for dm-structs.h

The include-guard for dm-structs.h was misspelled as __DT_STTUCTS.
Change it.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/dt-structs.h

@@ -4,8 +4,8 @@
  * SPDX-License-Identifier:	GPL-2.0+
  */
 
-#ifndef __DT_STTUCTS
-#define __DT_STTUCTS
+#ifndef __DT_STRUCTS
+#define __DT_STRUCTS
 
 /* These structures may only be used in SPL */
 #if CONFIG_IS_ENABLED(OF_PLATDATA)