DeviceKit-power ext.dep

[Matthias Clasen <matthias clasen gmail com>]

Hey, we currently have DeviceKit-power-009 in our external deps. This
has a dependency on libdevicekit-gobject from the deceased DeviceKit
module, causing my jhbuild to fail, since DeviceKit has already
vanished from Fedora.

I'd like to propose that we should either bump the DK-power dep to a
udev-based version, or treat it like DK-disks/udisks and just put it
on the 'system requirements' side and drop it from ext.deps.

Comments ?