arch/x86/include: Remove unused <stddef.h>

Change-Id: I2a4b00d06c92eea1b83002c69d93037f84592393
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72111
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/include/mode_switch.h

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <stddef.h>
 #include <stdint.h>
 
 #if ENV_X86_64

src/arch/x86/include/smm.h

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <stddef.h>
 #include <stdint.h>
 #include <cpu/x86/smm.h>