INSTALL_PATH_PREFIX is used even when not installing

INSTALL_PATH_PREFIX is used even when not installing
https://bugs.webkit.org/show_bug.cgi?id=120810

Reviewed by Andy Estes.

* Configurations/CopyWTFHeaders.xcconfig: Removed INSTALL_PATH_PREFIX from
the definition of PRIVATE_HEADERS_FOLDER_PATH. Also removed the leading
slash, because the Xcode definition of this build setting is relative.
* WTF.xcodeproj/project.pbxproj: Prepend INSTALL_PATH_PREFIX here when installing.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@155171 268f45cc-cd09-0410-ab3c-d52691b4dbfc