lib: memcpy_kunit: Fix an invalid format specifier in an assertion msg

The 'i' passed as an assertion message is a size_t, so should use '%zu',
not '%d'.

This was found by annotating the _MSG() variants of KUnit's assertions
to let gcc validate the format strings.

Fixes: bb95ebbe ("lib: Introduce CONFIG_MEMCPY_KUNIT_TEST")
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>