platform/chrome: cros_usbpd_logger: Add __printf annotation to append_str()

This allows the compiler to verify the format strings vs the types of
the arguments. Also, silence the warning (triggered by W=1):

  cros_usbpd_logger.c:55:2: warning: function ‘append_str’ might be a
  candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>