src: Remove unused 'include <string.h>'

Change-Id: I2a94c3b6282e9915fd2b8136b124740c8a7b774c
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36082
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/superio/aspeed/ast2400/superio.c

@@ -18,7 +18,6 @@
 #include <device/device.h>
 #include <device/pnp.h>
 #include <superio/conf_mode.h>
-#include <string.h>
 #include <pc80/keyboard.h>
 #include <superio/common/ssdt.h>
 #include <arch/acpi.h>