power: supply: Fix build error when CONFIG_POWER_SUPPLY is not enabled.

The build error happens when CONFIG_POWER_SUPPLY is not enabled.

h8300-linux-ld: drivers/usb/dwc3/gadget.o: in function `.L59':
>> gadget.c:(.text+0x655): undefined reference to `power_supply_set_property'

Fixes: 99288de3 ("usb: dwc3: add an alternate path in vbus_draw callback")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ray Chi <raychi@google.com>
Link: https://lore.kernel.org/r/20210327182809.1814480-3-raychi@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/power_supply.h

@@ -426,9 +426,16 @@ static inline int power_supply_is_system_supplied(void) { return -ENOSYS; }
 extern int power_supply_get_property(struct power_supply *psy,
 			    enum power_supply_property psp,
 			    union power_supply_propval *val);
+#if IS_ENABLED(CONFIG_POWER_SUPPLY)
 extern int power_supply_set_property(struct power_supply *psy,
 			    enum power_supply_property psp,
 			    const union power_supply_propval *val);
+#else
+static inline int power_supply_set_property(struct power_supply *psy,
+			    enum power_supply_property psp,
+			    const union power_supply_propval *val)
+{ return 0; }
+#endif
 extern int power_supply_property_is_writeable(struct power_supply *psy,
 					enum power_supply_property psp);
 extern void power_supply_external_power_changed(struct power_supply *psy);