/usr/lib/x86_64-linux-gnu/libvarnishapi.so is in libvarnishapi-dev 5.2.1-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libvarnishapi.so.1.0.6
/usr/lib/x86_64-linux-gnu/libvarnishapi.so.1.0.6