HID: steam: select CONFIG_POWER_SUPPLY

Using the power supply APIs requires selecting the appropriate
Kconfig symbol, otherwise we get this build failure:

drivers/hid/hid-steam.o: In function `steam_unregister':
hid-steam.c:(.text+0x1cc): undefined reference to `power_supply_unregister'
drivers/hid/hid-steam.o: In function `steam_battery_get_property':
hid-steam.c:(.text+0x2d2): undefined reference to `power_supply_get_drvdata'
drivers/hid/hid-steam.o: In function `steam_raw_event':
hid-steam.c:(.text+0xcba): undefined reference to `power_supply_changed'
drivers/hid/hid-steam.o: In function `steam_register':
hid-steam.c:(.text+0x13e3): undefined reference to `power_supply_register'
hid-steam.c:(.text+0x13fe): undefined reference to `power_supply_powers'

Fixes: f8271979 ("HID: steam: add battery device.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>