src/ec: Use 'include <stdlib.h>' when appropriate

Change-Id: Ifdb2dee08da45d698174583ee5ed44bf5a0243ff
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32823
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/ec/google/wilco/mailbox.c

@@ -18,7 +18,6 @@
 #include <delay.h>
 #include <ec/google/common/mec.h>
 #include <stdint.h>
-#include <stdlib.h>
 #include <string.h>
 #include <timer.h>
 #include <types.h>