util/cbfstool/cbfs-mkstage.c: Fix build issue on 32-bit x86

Fixes regression caused by commit 405304ac
(cbfstool: Add option to ignore section in add-stage)

Change-Id: If9e3eea9ab2c05027f660d0057a635abf981b901
Signed-off-by: Francis Rowe <info@gluglug.org.uk>
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7545
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>