/usr/lib/ is in ruby-fftw3-dbg 0.4-6.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/debug/
/usr/lib/debug/usr/
/usr/lib/debug/usr/lib/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/ruby/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.0.0/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.0.0/numru/
/usr/lib/debug/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.0.0/numru/fftw3.so
/usr/lib/debug/usr/lib/ruby/
/usr/lib/debug/usr/lib/ruby/vendor_ruby/
/usr/lib/debug/usr/lib/ruby/vendor_ruby/1.9.1/
/usr/lib/debug/usr/lib/ruby/vendor_ruby/1.9.1/x86_64-linux/
/usr/lib/debug/usr/lib/ruby/vendor_ruby/1.9.1/x86_64-linux/numru/
/usr/lib/debug/usr/lib/ruby/vendor_ruby/1.9.1/x86_64-linux/numru/fftw3.so