ARM: SAMSUNG: Fix compile error due to kfree

Fix following build error.
arch/arm/plat-samsung/dev-backlight.c: In function 'samsung_bl_set':
arch/arm/plat-samsung/dev-backlight.c:145: error: implicit declaration of function 'kfree'
Signed-off-by: NJonghwan Choi <jhbird.choi@gmail.com>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>

arch/arm/plat-samsung/dev-backlight.c

@@ -14,6 +14,7 @@
 #include <linux/platform_device.h>
 #include <linux/io.h>
 #include <linux/pwm_backlight.h>
+#include <linux/slab.h>
 
 #include <plat/devs.h>
 #include <plat/gpio-cfg.h>