/usr/lib/x86_64-linux-gnu/libfftw3f_threads.so is in libfftw3-dev 3.3.3-7ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfftw3f_threads.so.3.3.2
/usr/lib/x86_64-linux-gnu/libfftw3f_threads.so.3.3.2