For out of source builds of Qt, the private headers are in
QT_BUILD_TREE (opposed to QT_INSTALL_PREFIX, or QT_SOURCE_TREE)

Change-Id: I3f22fdefffd43df4c71098425de4e44f561e6ab1
Reviewed-on: http://codereview.qt.nokia.com/900


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>