src/include: Remove unused 'include <stddef.h>'

Change-Id: I525eb58669d256286e8476b12174d37d1d9aa3bf
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43370
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/list.h

@@ -4,8 +4,6 @@
 #ifndef __LIST_H__
 #define __LIST_H__
 
-#include <stddef.h>
-
 struct list_node {
 	struct list_node *next;
 	struct list_node *prev;

src/include/thread.h

@@ -2,7 +2,6 @@
 #ifndef THREAD_H_
 #define THREAD_H_
 
-#include <stddef.h>
 #include <stdint.h>
 #include <bootstate.h>
 #include <arch/cpu.h>