dm: core: Guard against including dm.h in header files

Header files generally should not include header files just for a struct,
since forward declarations work just as well and reduce overhead.

Add a warning for dm.h being included, since this has crept into U-Boot.
Signed-off-by: NSimon Glass <sjg@chromium.org>

include/dm.h

@@ -3,6 +3,10 @@
  * Copyright (c) 2013 Google, Inc
  */
 
+#ifdef _DM_H_
+#warning "Suspect dm.h is included from a header file - please fix"
+#endif
+
 #ifndef _DM_H_
 #define _DM_H_