docs: Properly mark acl.html dependencies
    
    The acl.html file includes aclperms.htmlinc which is generated.
    However, acl.html is generated too from acl.html.tmp. And in fact,
    this is the place where the aclperms file is needed. Fix the
    dependency in Makefile.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>