cgpt: Fix format specifier for size_t

Use %zu, which is the format specifier for size_t.

BUG=none
TEST=make clean && make runtests
BRANCH=none

Change-Id: I3317c2f6a7b9d95c22a43ae3d786c7d7380342ad
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 199882
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1737706
Reviewed-by: Julius Werner <jwerner@chromium.org>