[SECURITY] Fedora 26 Update: heimdal-7.4.0-1.fc26

Kerberos 5 is a network authentication and single sign-on system. Heimdal is a free Kerberos 5 implementation without export restrictions written from the spec (rfc1510 and successors) including advanced features like thread safety, IPv6, master-slave replication of Kerberos Key Distribution Center server and support for ticket delegation (S4U2Self, S4U2Proxy). This package can coexist with MIT Kerberos 5 packages. Hesiod is disabled by default since it is deemed too big a security risk by the packager.