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>

drivers/base/power/domain.c

@@ -159,7 +159,7 @@ static void genpd_recalc_cpu_exit_latency(struct generic_pm_domain *genpd)
  * Restore power to @genpd and all of its masters so that it is possible to
  * resume a device belonging to it.
  */
-int __pm_genpd_poweron(struct generic_pm_domain *genpd)
+static int __pm_genpd_poweron(struct generic_pm_domain *genpd)
 	__releases(&genpd->lock) __acquires(&genpd->lock)
 {
 	struct gpd_link *link;