Update libudev URL.
Update libudev URL.

The old URL has been removed; the content moved to
http://www.freedesktop.org/software/systemd/man/libudev.html .
However, now that libudev has been absorbed by systemd, set the URL
to the less volatile systemd URL.

BUG=chromium:476190

Review URL: https://codereview.chromium.org/1392343003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353585}

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