build-sys: let libsystemd_network pull in libudev-internal.la

On Thu, May 22, 2014 at 9:53 AM, Jan Engelhardt <jengelh@inai.de> wrote:
>
> If libsystemd-network.la is relying on that udev function, it ought
> to specify libudev(-internal).la in libsystemd_network_la_LIBADD.