leds: lgm: fix gpiolib dependency

leds: lgm: fix gpiolib dependency

Without gpiolib, the driver fails to build:

    drivers/leds/blink/leds-lgm-sso.c:123:19: error: field has incomplete type 'struct gpio_chip'
            struct gpio_chip chip;
                             ^
    include/linux/gpio.h:107:8: note: forward declaration of 'struct gpio_chip'
    struct gpio_chip;
           ^
    drivers/leds/blink/leds-lgm-sso.c:263:3: error: implicit declaration of function 'gpiod_set_value' [-Werror,-Wimplicit-function-declaration]
                    gpiod_set_value(led->gpiod, val);
                    ^
    drivers/leds/blink/leds-lgm-sso.c:263:3: note: did you mean 'gpio_set_value'?
    include/linux/gpio.h:168:20: note: 'gpio_set_value' declared here
    static inline void gpio_set_value(unsigned gpio, int value)
                       ^
    drivers/leds/blink/leds-lgm-sso.c:345:3: error: implicit declaration of function 'gpiod_set_value' [-Werror,-Wimplicit-function-declaration]
                    gpiod_set_value(led->gpiod, 1);
                    ^

Add the dependency in Kconfig.

Fixes: c3987cd2bca3 ("leds: lgm: Add LED controller driver for LGM SoC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pavel Machek <pavel@ucw.cz>