tc: m_xt: Fix segfault with iptables-1.6.0

tc: m_xt: Fix segfault with iptables-1.6.0

Said iptables version introduced struct xtables_globals field
'compat_rev', a function pointer. Initializing it is mandatory as
libxtables calls it without existence check.

Without this, tc segfaults when using the xt action like so:

| tc filter add dev d0 parent ffff: u32 match u32 0 0 \
|	action xt -j MARK --set-mark 20

Signed-off-by: Phil Sutter <phil@nwl.cc>