/usr/lib/x86_64-linux-gnu/libknot.so is in libknot-dev 2.4.0-3+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libknot.so.5.0.0
/usr/lib/x86_64-linux-gnu/libknot.so.5.0.0