src: work with typeof

Although not officially supported, we have found that the
xtables-addons modules we are interested in work fine on 3.0.4 with a
slight non-invasive mod to compat_xtables.h.
xt_pknock: fix pknock in UDP SPA mode

When the PK_CRYPTO pre-processor flag got removed in
v1.47.1-2-g66f213e, one of the removal cases was misapplied; the body
of an "#ifndef PK_CRYPTO" was left in rather than the whole section
being removed.
build: explicitly support Linux 3.16, 3.17