Put xtables.c into its own library, libxtables.so

Put xtables.c into its own library, libxtables.so

iproute2's tc's m_ipt.c poses as a pseudo-iptables program to make
use of the info structure composition of iptables extensions.
Since tc would have to clone a lot of code, xtables.c is put into
its own shared library and should not be relied upon by any other
programs.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>