/usr/include/krb5/hostrealm_plugin.h is in libkrb5-dev 1.15-1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/krb5/../mit-krb5/krb5/hostrealm_plugin.h
/usr/include/krb5/../mit-krb5/krb5/hostrealm_plugin.h