util/ectool: Fix compiler warning
util/ectool: Fix compiler warning

When compiling with C++, the original code generates the following
error:

error: initializer-string for char array is too long
.buf = "0123456789abcdef0123456789ABCDEF"

BRANCH=none
BUG=b:144959033
TEST=make buildall

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: Ibf599d72d6c9308f10131edfb5a796a416574e6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2864517
Reviewed-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec