hwmon: (sht15) Fix Kconfig dependencies

sht15 depends on GPIOLIB, not on GENERIC_GPIO.

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

src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_direction_input': => 293:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_direction_output': => 216:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_free': => 1000:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_get_value': => 296:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_request': => 946:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_set_value': => 218:2
src/drivers/hwmon/sht15.c: error: implicit declaration of function 'gpio_to_irq': => 514:2

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