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: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NSebastian Reichel <sre@kernel.org>