contrib/wix/defines.wxi

bugzilla: fix reST error introduced in 22d200e49b10

All blocks must be indented. Here we can use a field list instead of a
verbatim block for a real list layout.

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>