libgudev: New package

As libgudev recently was split from the main systemd/udev source, this
library is now required to build certain packages.

This library is only relevant to systemd, as the code it contains is
still contained in eudev.

[Thomas:
 - don't show the dependency comment when systemd is not available,
   since libgudev is anyway useless when you're not using systemd.
 - fix the license, it's LGPLv2.1+ and not GPLv2+
 - remove useless empty lines in the .mk file.]

Signed-off-by: Nathaniel Roach <nroach44@gmail.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>