/usr/lib/x86_64-linux-gnu/librt.so is in libc6-dev 2.24-11+deb9u4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /lib/x86_64-linux-gnu/librt.so.1
/lib/x86_64-linux-gnu/librt.so.1