iptables: add parallel build fix patch

Fixes:
http://autobuild.buildroot.net/results/5c4/5c4ca595c4bf744aa00fc58851eeccf81f9c6ecc/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>