iproute2: disable xtables on static build

iproute2 always builds m_xt.so as a shared library.

Fixes:
http://autobuild.buildroot.net/results/f42/f42bbbf5be4594a8806d0e571586ce2bf119582d/
http://autobuild.buildroot.net/results/3b3/3b37871bdf5766677fc20dca22c13177091d104f/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>