dtc: Include stdlib.h in util.h

If used on its own, util.h needs stdlib.h for exit(), malloc() and
realloc().

Signed-off-by: Andrei Ziureaev <andrei.ziureaev@arm.com>
Message-Id: <20200721155900.9147-2-andrei.ziureaev@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>