thermal: Change IS_ENABLED to IFDEF in the header file

The thermal framework can not be compiled as a module. The IS_ENABLED
macro is useless here and can be replaced by an ifdef.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-7-daniel.lezcano@linaro.org

include/linux/thermal.h

@@ -383,7 +383,7 @@ void devm_thermal_zone_of_sensor_unregister(struct device *dev,
 
 #endif
 
-#if IS_ENABLED(CONFIG_THERMAL)
+#ifdef CONFIG_THERMAL
 struct thermal_zone_device *thermal_zone_device_register(const char *, int, int,
 		void *, struct thermal_zone_device_ops *,
 		struct thermal_zone_params *, int, int);