Fix build error when CONFIG_THERMAL=m and SENSORS_GPIO_FAN=y
by preventing that combination.

Fixes these build errors:

drivers/built-in.o: In function `gpio_fan_remove':
gpio-fan.c:(.text+0x21e97e): undefined reference to `thermal_cooling_device_unregister'
drivers/built-in.o: In function `gpio_fan_probe':
gpio-fan.c:(.text+0x21efbc): undefined reference to `thermal_cooling_device_register'
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	Jean Delvare <jdelvare@suse.de>
Cc:	Guenter Roeck <linux@roeck-us.net>
Cc:	lm-sensors@lm-sensors.org
Cc:	Simon Guinot <sguinot@lacie.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>