ARM: SAMSUNG: remove duplicated header include

This patch removes duplicated slab header for pwm backlight.
arch/arm/plat-samsung/dev-backlight.c: slab.h is included
more than once.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>