thermal: hwmon: EXPORT_SYMBOL_GPL for thermal hwmon sysfs

thermal_add_hwmon_sysfs()/thermal_remove_hwmon_sysfs() need
EXPORT_SYMBOL_GPL(). Otherwise we will have ERROR

>> ERROR: "thermal_remove_hwmon_sysfs" [drivers/thermal/rcar_thermal.ko] undefined!
>> ERROR: "thermal_add_hwmon_sysfs" [drivers/thermal/rcar_thermal.ko] undefined!

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>