drivers/video/backlight/aat2870_bl.c: make it buildable as a module

i386 allmodconfig:

  drivers/built-in.o: In function `aat2870_bl_remove':
  aat2870_bl.c:(.text+0x414f9): undefined reference to `backlight_device_unregister'
  drivers/built-in.o: In function `aat2870_bl_probe':
  aat2870_bl.c:(.text+0x418fc): undefined reference to `backlight_device_register'
  aat2870_bl.c:(.text+0x41a31): undefined reference to `backlight_device_unregiste

Cc: Jin Park <jinyoungp@nvidia.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Axel Lin <axel.lin@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>