tc: let m_ipt work with new iptables API headers

tc: let m_ipt work with new iptables API headers

Since commit 5cd1adb ("Update to current iptables headers") the build
with m_ipt.o and the following config will fail:

TC_CONFIG_XT:=n
TC_CONFIG_XT_OLD:=n
TC_CONFIG_XT_OLD_H:=n

This patch renames "iptables_target" to "xtables_target" and some other
things which gets renamed and I noticed while reading iptables git log.
Functions which are not used in m_ipt.c and not exported by the header
are removed, if they still used in m_ipt.c I added a static to the function.

Reported-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>