PM / Sleep: Add missing static storage class specifiers in main.c

Fix the following sparse warnings:
drivers/base/power/main.c:48:1: warning: symbol 'dpm_prepared_list' was not declared. Should it be static?
drivers/base/power/main.c:49:1: warning: symbol 'dpm_suspended_list' was not declared. Should it be static?
drivers/base/power/main.c:50:1: warning: symbol 'dpm_late_early_list' was not declared. Should it be static?
drivers/base/power/main.c:51:1: warning: symbol 'dpm_noirq_list' 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/main.c

@@ -45,10 +45,10 @@ typedef int (*pm_callback_t)(struct device *);
  */
 
 LIST_HEAD(dpm_list);
-LIST_HEAD(dpm_prepared_list);
-LIST_HEAD(dpm_suspended_list);
-LIST_HEAD(dpm_late_early_list);
-LIST_HEAD(dpm_noirq_list);
+static LIST_HEAD(dpm_prepared_list);
+static LIST_HEAD(dpm_suspended_list);
+static LIST_HEAD(dpm_late_early_list);
+static LIST_HEAD(dpm_noirq_list);
 
 struct suspend_stats suspend_stats;
 static DEFINE_MUTEX(dpm_list_mtx);