Reindent all XSD files for consistency

[petterreinholdtsen]

This make it easier to compare the files across versions, as well as
improves readability.

The update was done using

  for x in $(find . -name '*.xsd'); do
    xmllint --format "$x" > "$x.new" && mv "$x.new" "$x"
  done

on Debian GNU/Linux, but any system with xmllint available will do.