libnftnl: add patch to avoid symbol conflict

When ELF binaries and shared libraries are used, the internal
functions of libnftnl such as xfree() are not visible to the outside
world (their visibility is 'hidden'). Therefore, the fact that other
programs (especially nftables) may have symbols with the same name
does not cause any problem.

However, when doing static linking on a non-ELF platform (such as
Blackfin, which uses the FLAT binary format), there is no way of
encoding this visibility. Therefore, the xfree() symbols of libnftnl
becomes visible to the outside world, causing a conflict with the
xfree() symbol defined by nftables.

To solve this, this patch renames the libnftnl xfree() function to
libnftnl_xfree().

This commit fixes a problem seen after building nftables statically on
Blackfin. This problem was seen after the previous patch fixing the
autobuilder failure:

  http://autobuild.buildroot.org/results/98b/98b707ffdeeb1cda94b7c1019ef29cf5fd7db8bf/



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