hwmon: (gpio-fan) Fix Kconfig dependencies

gpio-fan depends on GPIOLIB, not on GENERIC_GPIO.

This fixes the following build error, seen if GPIOLIB is not defined:

src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_direction_output': => 372:3
src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_free': => 130:2
src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_get_value': => 79:2
src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_request': => 98:2
src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_set_value': => 156:3
src/drivers/hwmon/gpio-fan.c: error: implicit declaration of function 'gpio_to_irq': => 114:2

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Simon Guinot <sguinot@lacie.com>