contrib/wix/defines.wxi

logtoprocess: do not use platform.system()

See the previous patch for the reason.

Differential Revision: https://phab.mercurial-scm.org/D1019

<Include>

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

</Include>