hwmon: (stts751) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/stts751.c:88:34: warning:
	‘stts751_of_match’ defined but not used

Mark stts751_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/stts751.c

@@ -85,7 +85,7 @@ static const struct i2c_device_id stts751_id[] = {
 	{ }
 };
 
-static const struct of_device_id stts751_of_match[] = {
+static const struct of_device_id __maybe_unused stts751_of_match[] = {
 	{ .compatible = "stts751" },
 	{ },
 };