/usr/lib/x86_64-linux-gnu/libgtksourceviewmm-3.0.so is in libgtksourceviewmm-3.0-dev 3.18.0-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgtksourceviewmm-3.0.so.0.0.0
/usr/lib/x86_64-linux-gnu/libgtksourceviewmm-3.0.so.0.0.0