dm: core: Include dt-decl.h automatically

When dt-structs.h is used, include the dt-decl.h header as well, so that
these declarations are available.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dt-structs.h b/include/dt-structs.h
index f0e1c9c..f9ccaf5 100644
--- a/include/dt-structs.h
+++ b/include/dt-structs.h
@@ -24,7 +24,9 @@
 	uint idx;
 	int arg[2];
 };
+
 #include <generated/dt-structs-gen.h>
+#include <generated/dt-decl.h>
 #endif
 
 #endif