broken nouveau dependency on power supply

[Benjamin Herrenschmidt]

Hi folks !

With CONFIG_POWER_SUPPLY=m & nouveau built-in we get a build failure:

drivers/built-in.o: In function `.nouveau_pm_trigger':
(.text+0xa56e8): undefined reference to `.power_supply_is_system_supplied'

nouveau probably needs to depends on CONFIG_POWER_SUPPLY to force a module
build with the latter is =m

Cheers,
Ben.