lib:krb5_wrap: Implement smb_krb5_principal_is_tgs()

This will be used later and allows to remove static implementations.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
