[power supply]:missing definition without config

[fuyao]

linux-pm:
hi, walk around the power_supply.h file. 
call power_supply_get_by_name without the CONFIG_POWER_SUPPLY, cause
error.
if we should use:

#ifdef CONFIG_POWER_SUPPLY
extern extern struct power_supply *power_supply_get_by_name(const char *name);
#else
static inline struct power_supply *
power_supply_get_by_name(const char *name)
{return NULL;}
#endif