PM / Domains: Skip governor functions for CONFIG_PM_RUNTIME unset

The governor functions in drivers/base/power/domain_governor.c
are only used if CONFIG_PM_RUNTIME is set and they refer to data
structures that are only present in that case.  For this reason,
they shouldn't be compiled at all when CONFIG_PM_RUNTIME is not set.

Reported-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>