iproute2: fix em_ipset build issue

Fixes http://autobuild.buildroot.net/results/be19e39c19b62946ea48dd6b3dfd55f5c3ccd7c9



em_ipset.c uses xtables.h from iptables, but the configure script fails
to to check for this, so forcibly disable it if not available.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>