AUTOMAKE: Do not include generated files into tarball
    
    sssd.service was assigned to the dist_systemdunit_DATA variable.
    Automake will install this file into the systemd unit directory after building
    it if necessary. Automake will also include this generated file in the tarball.
    As a result, when building sssd from the tarball, the paths needn't be
    recreated.
    
    The files in DATA primaries are added as dependencies to the all target
    via the internal all-am target. If sssd.service doesn’t exist, make will look
    for a rule to build it. Since there is such a rule, make will simply execute
    that rule when I build the all target.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2314
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>