TriState.h should be a Project header
https://bugs.webkit.org/show_bug.cgi?id=110543

Rubber-stamped by Dan Bernstein.

WTF uses a script to copy headers to the build destination, so it isn't
necessary to mark headers as Private/Public (and doing so can be problematic).

* WTF.xcodeproj/project.pbxproj: Mark TriState.h as a Project header.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143682 268f45cc-cd09-0410-ab3c-d52691b4dbfc