staging: olpc_dcon: add a missing dependency

Commit 33f49571 upstream.

  WARNING: unmet direct dependencies detected for BACKLIGHT_CLASS_DEVICE
    Depends on [n]: HAS_IOMEM [=y] && BACKLIGHT_LCD_SUPPORT [=n]
    Selected by [y]:
    - FB_OLPC_DCON [=y] && STAGING [=y] && X86 [=y] && OLPC [=y] && FB [=y]
                        && I2C [=y] && (GPIO_CS5535 [=n] || GPIO_CS5535 [=n]=n)

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>