tree: Remove unused <stddef.h>

Change-Id: I7d7ad562eeff7247b7377b6570d489faee0aeda0
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82669
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yidi Lin <yidilin@google.com>

src/ec/google/chromeec/ec_skuid.c

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
-#include <stddef.h>
 #include <ec/google/chromeec/ec.h>
 
 uint32_t google_chromeec_get_board_sku(void)

src/ec/google/chromeec/ec_smbios.c

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
-#include <stddef.h>
 #include <ec/google/chromeec/ec.h>
 #include <console/console.h>
 #include <string.h>

src/ec/purism/librem-ec/librem_ec.c

@@ -1,8 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <commonlib/bsd/helpers.h>
 #include "librem_ec.h"
 #include "../../system76/ec/system76_ec.h"
-#include <stddef.h>
 
 #define CMD_PROBE 1