thermal/hwmon: Use the thermal_core.h header

The thermal_hwmon is playing with the thermal core code
internals. Changing the code would be too invasive for now.

We can consider the thermal_hwmon.c is part of the thermal core code
as it provides a glue to tie the hwmon and the thermal zones.

Let's include the thermal_core.h header.

No functional change intended.

Cc: Jean Delvare <jdelvare@suse.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>