package/network-manager: now needs libgudev when systemd is used

NetworkManager uses code (originally from udev) that has since been
split from the main systemd codebase into libgudev.

Tweak the package files for NetworkManager to require libgudev when
building with systemd.

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