src: replace IPTABLES_VERSION by PACKAGE_VERSION

src: replace IPTABLES_VERSION by PACKAGE_VERSION

The IPTABLES_VERSION C macro replicates the PACKAGE_VERSION C macro
(both have the same definition, "@PACKAGE_VERSION@"). Since
IPTABLES_VERSION, being located in internal.h, is not exposed to
downstream users in any way, it can just be replaced by
PACKAGE_VERSION, which saves a configure-time file substitution.
This goes towards eliminating unnecessary rebuilds after rerunning
./configure.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Florian Westphal <fw@strlen.de>