SPEC: Workaround for build with rpm 4.13
    
    If the tarball is generated with minimal dependencies extracted from spec file
    then translated manual pages are not generated due to missing script po4a.
    This step is not necessary for regular nightly/developer builds.
    The tarball is created faster without such step. However rpm >= 4.13
    will fail due to empty manifest file.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2738
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>