REGRESSION (r176683): RefCounter.h is installed outside SDKROOT

* WTF.xcodeproj/project.pbxproj: Don't mark RefCounter.h as a
private header.  WTF headers use a special build phase script to
be installed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176832 268f45cc-cd09-0410-ab3c-d52691b4dbfc