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>
Cc: stable@kernel.org

drivers/platform/x86/Kconfig

@@ -390,6 +390,7 @@ config EEEPC_WMI
 	depends on ACPI_WMI
 	depends on INPUT
 	depends on EXPERIMENTAL
+	depends on BACKLIGHT_CLASS_DEVICE
 	select INPUT_SPARSEKMAP
 	---help---
 	  Say Y here if you want to support WMI-based hotkeys on Eee PC laptops.