/usr/include/gssapi/gssapi_ext.h is in libkrb5-dev 1.16-2build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/gssapi/../mit-krb5/gssapi/gssapi_ext.h
/usr/include/gssapi/../mit-krb5/gssapi/gssapi_ext.h