dbus: change 'system-release' dependency to 'fedora-release'
    
    As it turns out, 'fedora-release' only has:
    
        'Provides: system-release'
    
    rather than:
    
        'Provides: system-release = %{version}-%{release}'
    
    This means, any explicit dependencies on 'system-release' ignore
    versioning completely. This is unfortunate, and breaks our reliance on
    'system-release'.
    
    Fortunately, unlike recommended on the wiki, it is safe to depend on
    'fedora-release' explicitly. Other fedora flavors have this package as
    well, so there is no reason not to.
    
    Signed-off-by: David Herrmann <dh.herrmann@gmail.com>