/usr/lib/x86_64-linux-gnu/librtmidi.so is in librtmidi-dev 1.0.15~ds0-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/librtmidi.so.1
/usr/lib/x86_64-linux-gnu/librtmidi.so.1