Fixes problems with xtables based MARK target ("ipt" module).
When tc loads the "ipt" (xt) module it kept the symbols local,
this made loading of libxtables not find the required struct.

currently ipt/xt is the only tc action module.
iproute2 never seem to do dlclose.
hopefully the modules doesn't export more symbols then needed.

In this situation hopefully the RTLD_GLOBAL flag won't hurt us.

I've been using this patch in the Debian package of iproute for
the last 3 weeks and noone has complained.
( This fixes http://bugs.debian.org/584898 )
Signed-off-by: Andreas Henriksson <andreas@fatal.se>