eeepc-wmi: depends on BACKLIGHT_CLASS_DEVICE

eeepc-wmi uses backlight*() interfaces so it should depend on
BACKLIGHT_CLASS_DEVICE.

eeepc-wmi.c:(.text+0x2d7f54): undefined reference to `backlight_force_update'
eeepc-wmi.c:(.text+0x2d8012): undefined reference to `backlight_device_register'
eeepc-wmi.c:(.devinit.text+0x1c31c): undefined reference to `backlight_device_unregister'
eeepc-wmi.c:(.devexit.text+0x2f8b): undefined reference to `backlight_device_unregister'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>