strongswan: disable iptables support

strongswan: disable iptables support

There is a new conflict between Linux header (linux/if.h)
and C library header (net/if.h) introduced by this commit
to the Linux kernel:
1ffad83dffd675cd742286ae82dca7d746cb0da8
Mikko Rapeli is working on a solution, but it requires
changes to the Linux kernel and C library.

For now I would just disable the iptables feature in Strongswan.

The changes fixes following autobuild error:
http://autobuild.buildroot.net/results/e1693a714eb690988c1798b733d0ff143e50b3b9/

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>