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