net-firewall/fwknop: re-enable in-source build wrt bug 568958

net-firewall/fwknop: re-enable in-source build wrt bug 568958

Otherwise Python C extension fails to build because it links against fko
library built during src_compile stage. With out-of-source build this library
resides in a separate directory from sources, thus it cannot be found during
Python module compilation by normal means.

This problem didn't catch my attention earlier because build failure is only
reproducible if there is no fko library installed in the system from a previous
version of this package.

Gentoo-Bug: 568958

Package-Manager: portage-2.2.24