/usr/lib/x86_64-linux-gnu/libXdmcp.so is in libxdmcp-dev 1:1.1.2-1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXdmcp.so.6.0.0
/usr/lib/x86_64-linux-gnu/libXdmcp.so.6.0.0