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>