package/iptables: fix static build

package/iptables: fix static build

Static build of iptables 1.8.5 fails on:

/srv/storage/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/8.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: ../libxtables/.libs/libxtables.a(libxtables_la-xtables.o): in function `dlreg_free':
/srv/storage/autobuild/run/instance-3/output-1/build/iptables-1.8.5/libxtables/xtables.c:235: undefined reference to `dlclose'

Issue has been added with
https://git.netfilter.org/iptables/commit/?id=7db4333dc0b6cd8e943fab78d6dab40ff9f4512e

Fixes:
 - http://autobuild.buildroot.org/results/00f551be87507757a78e332f2859b1e7b3190363

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>