Get udevrulesdir from pkg-config

udev upstream likes to move this around
(they just did it again, during the udev -> systemd merge)

But they provide a standard way to find the dir through pkg-config.
This has been around for ages, so use it.

https://github.com/hughsie/colord/commit/d1dbea3c58f7f69a55c15b2e05ef8e0622f71503