hwmon: (pmbus/ibm-cffps) Add dependency on LEDS_CLASS

Building without CONFIG_LEDS_CLASS causes a link failure:

drivers/hwmon/pmbus/ibm-cffps.o: In function `ibm_cffps_probe':
ibm-cffps.c:(.text+0x4f4): undefined reference to `devm_of_led_classdev_register'

This adds the required dependency.

Fixes: ef9e1cdf ("hwmon: (pmbus/cffps) Add led class device ...")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>