power_supply: Fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/power/power_supply_core.c:540:5: warning:
symbol '__power_supply_register' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/power_supply_core.c

···
 }
 #endif
 
-int __power_supply_register(struct device *parent, struct power_supply *psy, bool ws)
+static int __power_supply_register(struct device *parent,
+				   struct power_supply *psy, bool ws)
 {
 	struct device *dev;
 	int rc;