Always build the iptables lock when building iptables.

We have a build target for it, but it is not built by default. We
did not catch this in testing because:

1. Any device that mounts /system read-write (such as a device
   used for development) will not enounter this problem because
   iptables creates the file if it does not exist.
2. Running "mmm external/iptables" before flashing will build the
   lock.

Bug: 36826873
Test: lock=$OUT/system/etc/xtables.lock && rm -f $lock && make -j64 iptables && ls -l $lock
Test: deleting the lock and rebuilding/flashing causes the lock to exist on device
Change-Id: I15a85c02e990f347031accdd29cc3e831ec631c2
1 file changed