hwmon: Fix SENSORS_LM75 dependencies to eliminate build errors

Based on an earlier attempt by Randy Dunlap.

Fix SENSORS_LM75 dependencies to eliminate build errors:

drivers/built-in.o: In function `lm75_remove':
lm75.c:(.text+0x12bd8c): undefined reference to `thermal_zone_of_sensor_unregister'
drivers/built-in.o: In function `lm75_probe':
lm75.c:(.text+0x12c123): undefined reference to `thermal_zone_of_sensor_register'

Add depends on THERMAL since that is what provides the
register/unregister functions above, but only if THERMAL_OF was
selected as this is an optional feature of the driver.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>