Fix symlink violation for $SNAP/bin/iptables-xml

The snap store upload was failing with "package contains external
symlinks: bin/iptables-xml". This is because $SNAP/bin/iptables-xml
was symlinked to /sbin/xtables-multi.

Configflags such as --binddir and --sbindir, among others, don't appear
to help. As a result, just manually copy $SNAP/sbin/xtables-multi to
$SNAP/bin/iptables-xml in the install scriptlet.

Change-Id: I85a2584add41d3e8bad84a4af3914333a05371f7

snapcraft.yaml

@@ -115,6 +115,8 @@ parts:
       - libnftnl-dev
     configflags:
       - --disable-nftables
+    install: |
+      cp -f $SNAPCRAFT_PART_INSTALL/sbin/xtables-multi $SNAPCRAFT_PART_INSTALL/bin/iptables-xml
   ipset:
     source: http://ipset.netfilter.org/ipset-6.30.tar.bz2
     plugin: autotools