[tproxy] Patching iptables via apt-build

[Matt Miller]

> > apt-get source iptables ...
>
> Thanks, your build instructions appeared to work, but ... The .deb
> that I build does not seem to know about
> /lib/iptables/libipt_tproxy.so or /lib/iptables/libipt_TPROXY.so.

I hacked extensions/Makefile, adding"tproxy" and "TPROXY" to the
definition of PF_EXT_SLIB.  I then reran "dpkg-buildpackage" and all is
now well.  So, I don't know if it's just expected that I'm supposed to
manually fix this Makefile, of if I missed some other step in the
patching process.  Let me know if I've done something I shouldn't have
done, but things appear to be working now.