Since version 3.2.5, signed tarballs are distributed via GitHub. See e.g.
release announcements:
https://lists.infradead.org/pipermail/libnl/2017-May/002313.html
The home page is somewhat outdated, but still hosts mailing lists and git mirror.
* gnu/packages/linux.scm (libnl): Update to 3.3.0.
[source]: Switch to new release URL.