Add gss_inquire_attrs_for_mech()
Since commit 030a4a03a ("Report inquire_attrs_For_mech mech failures") in
MIT krb5, NTLMSSP fallback within SPNEGO is no longer working. It seems
that providing a gss_inquire_attrs_for_mech() function is now mandatory.
Although it does seem that perhaps krb5 should be a little more forgiving
and just assume GSS_C_NO_OID_SET, fix it anyway.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>