The newly added power supply code fails to link when the power supply core
code is disabled:

drivers/hid/hid-asus.o: In function `asus_battery_get_property':
hid-asus.c:(.text+0x11de): undefined reference to `power_supply_get_drvdata'
drivers/hid/hid-asus.o: In function `asus_probe':
hid-asus.c:(.text+0x170c): undefined reference to `devm_power_supply_register'
hid-asus.c:(.text+0x1734): undefined reference to `power_supply_powers'
drivers/hid/hid-asus.o: In function `asus_raw_event':
hid-asus.c:(.text+0x1914): undefined reference to `power_supply_changed'

Select the subsystem from Kconfig as we do for other hid drivers already.

Fixes: 6311d329 ("HID: hid-asus: Add BT keyboard dock battery monitoring support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>