build: bump SONAME for libxtables

Commit v1.4.17-16-gefcdba4 updated structs in xtables.h, so age must
become 0 and vcurrent be increased. The latter has already happened in
v1.4.17-6-gd1e7922.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>