src: Remove unused 'include <stddef.h>

Change-Id: Iae1e875b466f8a195653d897efa1b297c61ad0a5
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41912
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/timer_queue.c

@@ -1,5 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
-#include <stddef.h>
+
 #include <timer.h>
 
 #define MAX_TIMER_QUEUE_ENTRIES 64