hwmon: (ina209) Fix build warning

If CONFIG_OF is not enabled, the following build warning is observed.

drivers/hwmon/ina209.c:590:34: warning:
	‘ina209_of_match’ defined but not used

Mark ina209_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ina209.c

@@ -587,7 +587,7 @@ static const struct i2c_device_id ina209_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, ina209_id);
 
-static const struct of_device_id ina209_of_match[] = {
+static const struct of_device_id __maybe_unused ina209_of_match[] = {
 	{ .compatible = "ti,ina209" },
 	{ },
 };