lenovo/t530/mainboard.c: Include header `h8.h` for prototype

Change-Id: I05cea020e77051bd3bc0e93b0c70e12b9b985d05
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6398
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>