libxtables: reorder .version member

libxtables: reorder .version member

When the structure's layout changes, as it did between v1.4.1 and
v1.4.2, trying to compare the version string makes iptables segfault
while it tries to determine whether the module is compatible in the
first place.

By moving the member to a known offset in the struct and keeping it
there, objects (both iptables and 3rd party) compiled from this
commit onwards will avoid the segfault.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>