/usr/lib/x86_64-linux-gnu/libpq.so is in libpq-dev 9.6.11-0+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libpq.so.5.9
/usr/lib/x86_64-linux-gnu/libpq.so.5.9