staging: fix olpc_dcon build, needs BACKLIGHT_CLASS_DEVICE

Fix olpc_dcon.c build by selecting the needed kconfig symbol
BACKLIGHT_CLASS_DEVICE.

olpc_dcon.c:(.text+0x11588b): undefined reference to `backlight_device_register'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Andres Salomon <dilinger@queued.net>
Cc: Chris Ball <cjb@laptop.org>
Cc: Jon Nettleton <jon.nettleton@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/olpc_dcon/Kconfig

@@ -2,6 +2,7 @@ config FB_OLPC_DCON
 	tristate "One Laptop Per Child Display CONtroller support"
 	depends on OLPC && FB
 	select I2C
+	select BACKLIGHT_CLASS_DEVICE
 	---help---
 	  Add support for the OLPC XO DCON controller.  This controller is
 	  only available on OLPC platforms.   Unless you have one of these