tools/leds: Add led_hw_brightness_mon program

LED subsystem supports POLLPRI on "brightness_hw_changed" sysfs file
of LED class devices. This tool demonstrates how to use the feature.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Pavel Machek <pavel@ucw.cz>