/usr/lib/x86_64-linux-gnu/libgtkmm-2.4.so is in libgtkmm-2.4-dev 1:2.24.5-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgtkmm-2.4.so.1.1.0
/usr/lib/x86_64-linux-gnu/libgtkmm-2.4.so.1.1.0