build-sys: fix (again) index generation in subdir

build-sys: fix (again) index generation in subdir

make-man-index.py doesn't care about .html files, only .xml files, so
the source list was wrong. Also, $(XML_FILES) are specified without
prefix, so compilation in sepearate build-dir was broken:

  GEN    man/index.html
Traceback (most recent call last):
  File "../make-man-index.py", line 24, in <module>
    t = parse(p)
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1183, in parse
    tree.parse(source, parser)
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 647, in parse
    source = open(source, "rb")
IOError: [Errno 2] No such file or directory: 'man/systemd.xml'