/usr/lib/x86_64-linux-gnu/libQt5Designer.so is in qttools5-dev 5.7.1-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libQt5Designer.so.5.7.1
/usr/lib/x86_64-linux-gnu/libQt5Designer.so.5.7.1