Add missing headers for compilation on x86 targets

Change-Id: I5f4dc797b353a0f01fa00d38375ec303bf863254
diff --git a/init/list.h b/init/list.h
index 0a7b28c..7b9ef32 100644
--- a/init/list.h
+++ b/init/list.h
@@ -17,6 +17,8 @@
 #ifndef _INIT_LIST_H_
 #define _INIT_LIST_H_
 
+#include <stddef.h>
+
 struct listnode
 {
     struct listnode *next;