thermal: hwmon: inline helpers when CONFIG_THERMAL_HWMON is not set

Avoid warnings like this:
thermal_hwmon.h:29:1: warning: ‘thermal_remove_hwmon_sysfs’ defined but not used [-Wunused-function]
 thermal_remove_hwmon_sysfs(struct thermal_zone_device *tz)

Fixes: 0dd88793 ("thermal: hwmon: move hwmon support to single file")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/thermal_hwmon.h

@@ -19,13 +19,13 @@
 int thermal_add_hwmon_sysfs(struct thermal_zone_device *tz);
 void thermal_remove_hwmon_sysfs(struct thermal_zone_device *tz);
 #else
-static int
+static inline int
 thermal_add_hwmon_sysfs(struct thermal_zone_device *tz)
 {
 	return 0;
 }
 
-static void
+static inline void
 thermal_remove_hwmon_sysfs(struct thermal_zone_device *tz)
 {
 }