/usr/lib/x86_64-linux-gnu/libXcomposite.so is in libxcomposite-dev 1:0.4.4-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXcomposite.so.1.0.0
/usr/lib/x86_64-linux-gnu/libXcomposite.so.1.0.0