platform/chrome: kunit: Avoid -Wflex-array-member-not-at-end

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the structure. Notice
that `struct cros_ec_command` is a flexible structure --a structure
that contains a flexible-array member.

Fix the following warning:

drivers/platform/chrome/cros_ec_proto_test_util.h:16:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/aBp7mZ8kj9w4CKkl@kspp


Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>