From the official announcement [1]:
IMPORTANT PLEASE NOTE
This release reverts an an uneeded SONAME bump that was done
in release 3.2.18 due to an ABI/API breakage in an unused
part of the API (object/cache API). After severe discussion,
it was decided to undo the SONAME bump to make life easier
for distribution maintainers and to remove the unused APIs
from the public header files.
If you linked against 3.2.18 or 3.2.19 your binary will not
be compatible with 3.2.20. You will have to relink your
binary. We are sorry for any inconvenience this may cause.
[1] http://lists.infradead.org/pipermail/libnl/2013-January/000910.html
