In some distros (such as Arch), libxtables_standard is part of iptables
as `/usr/lib/iptables/libxt_standard.so`. With this change, the test passes,
otherwise netns-nf test fails.

Tested with make test.
Signed-off-by: Raghavendra D Prabhu <me@rdprabhu.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>