build: generate libnl-idiag-3.0.pc

That file was not generated due to a missing line in Makefile.am
and configure.ac

http://lists.infradead.org/pipermail/libnl/2014-September/001649.html

Signed-off-by: Sébastien Barré <sebastien.barre@uclouvain.be>
Signed-off-by: Thomas Haller <thaller@redhat.com>