Exclude zlib 3rd party sources from our checks too.

zlib ChangeLog file is not in UTF-8 in 1.2.8 and so doesn't pass our checks.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74901 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775