/usr/lib/libevent_extra.so is in libevent-dev 2.0.16-stable-1ubuntu0.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libevent_extra-2.0.so.5.1.4
/usr/lib/libevent_extra-2.0.so.5.1.4