iptables plugin: fix implicit conversion warnings

iptables plugin: fix implicit conversion warnings

  CC       src/iptables_la-iptables.lo
src/iptables.c:139:29: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32]
  table_len = strlen(table) + 1;
            ~ ~~~~~~~~~~~~~~^~~
src/iptables.c:147:29: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32]
  chain_len = strlen(chain) + 1;
            ~ ~~~~~~~~~~~~~~^~~
2 warnings generated.