[WinCairo] Unreviewed build fix. Correct DropBox download link.

* Scripts/update-webkit-wincairo-libs: Revise download URL to
match change in DropBox server behavior.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148103 268f45cc-cd09-0410-ab3c-d52691b4dbfc