/usr/lib/x86_64-linux-gnu/libmysqlclient.so is in libmysqlclient-dev 5.5.54-0ubuntu0.12.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmysqlclient.so.18
/usr/lib/x86_64-linux-gnu/libmysqlclient.so.18