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 <[e-mail]>
Signed-off-by: Benson Leung <[e-mail]>

drivers/platform/chrome/cros_usbpd_logger.c

···
 	"---", "OCP", "fast OCP", "OVP", "Discharge"
 };
 
+__printf(3, 4)
 static int append_str(char *buf, int pos, const char *fmt, ...)
 {
 	va_list args;