libnftnl: add patch to fix libnftnl.pc for static linking

This commit adds a patch to libnftnl to add the Libs.private field in
the pkg-config file, which is needed for proper static linking of
applications using libnftnl.

Fixes the build of programs such as nftables on Blackfin:

  http://autobuild.buildroot.org/results/061/06166344cc8e162d3f901c70ee31ce07e481053e/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>