README: Fix directory where private headers can be found

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>

README

@@ -54,7 +54,7 @@ can fix this by either re-building your Qt with the "-developer-build"
 configure option, or by passing the include directory in the source directory
 to qmake. For example:
 
-  qmake "QT_PRIVATE_HEADERS=$$QT_SOURCE_TREE/include" $SOURCE_DIRECTORY/qtcreator.pro
+  qmake "QT_PRIVATE_HEADERS=$$QT_BUILD_TREE/include" $SOURCE_DIRECTORY/qtcreator.pro
 
 
 Compiling Qt and Qt Creator on Windows