leds: s3c24xx: Fix build failure

Commit c67d0f29 ("ARM: s3c24xx: get rid of custom <mach/gpio.h>")
removed the usage of mach/gpio.h file, but we need to include>
plat/gpio-cfg.h to avoid following build error.

Fixes following build error.
drivers/leds/leds-s3c24xx.c: In function ‘s3c24xx_led_probe’:
drivers/leds/leds-s3c24xx.c:100:2: error: implicit declaration of
function ‘s3c_gpio_setpull’ [-Werror=implicit-function-declaration]

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>