PM / Domains: Add missing static storage class specifier in domain.c file

Fixes the folloiwng sparse warning:
drivers/base/power/domain.c:149:5:
warning: symbol '__pm_genpd_poweron' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>