hwmon: (tmp421) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/tmp421.c:73:34: warning:
	‘tmp421_of_match’ defined but not used

Mark tmp421_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/tmp421.c

@@ -70,7 +70,7 @@ static const struct i2c_device_id tmp421_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, tmp421_id);
 
-static const struct of_device_id tmp421_of_match[] = {
+static const struct of_device_id __maybe_unused tmp421_of_match[] = {
 	{
 		.compatible = "ti,tmp421",
 		.data = (void *)2