Don't build the krb4 plugin as krb5 1.7 will drop it (#225974 #c6)
    This is temporary required because F12 is already shipping krb5 without
        krb4 support, thus rebuilding of cyrus-sasl in F12 without krb4 support
        fails as cyrus-sasl is part of the mock build root. I will enable this
        afterwards in F11 again to avoid F11 breakage and once F12 builds fit
        so far...