build: fix detection of systemd.pc
    
    Related: https://pagure.io/SSSD/sssd/issue/4043
    
    A typo prevents systemd.pc from being detected, in turn this means
    HAVE_SYSTEMD is never set, responders are built without socket
    activation support and the monitor never notifies systemd that it is
    ready.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>