hwmon: (ibmpowernv) Fix build error seen for some configurations

Fix
drivers/hwmon/ibmpowernv.c: In function 'get_logical_cpu':
drivers/hwmon/ibmpowernv.c:121:3:
	error: implicit declaration of function 'get_hard_smp_processor_id'

seen for some configurations, possibly if SMP is not configured.

Fixes: 3df2f59f ("hwmon: (ibmpowernv) pretty print labels")
Cc: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>