iptables: disable static for dynamic builds

It leads to ugly runtime warnings, see:
http://patchwork.ozlabs.org/patch/386215/


It's also required for xtables-addons.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>