/usr/lib/x86_64-linux-gnu/librtmp.so is in librtmp-dev 2.4+20151223.gitfa8646d-1ubuntu0.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/librtmp.so.1
/usr/lib/x86_64-linux-gnu/librtmp.so.1