src/include: Add missing includes

Change-Id: I746ea7805bae553a146130994d8174aa2e189610
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43368
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/device_tree.h

@@ -4,6 +4,7 @@
 #ifndef __DEVICE_TREE_H__
 #define __DEVICE_TREE_H__
 
+#include <stddef.h>
 #include <stdint.h>
 #include <list.h>