/usr/include/krb5/plugin.h is in libkrb5-dev 1.10+dfsg~beta1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/krb5/../mit-krb5/krb5/plugin.h
/usr/include/krb5/../mit-krb5/krb5/plugin.h