/usr/lib/x86_64-linux-gnu/libkj.so is in libcapnp-dev 0.5.3-2ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libkj-0.5.3.so
/usr/lib/x86_64-linux-gnu/libkj-0.5.3.so