[dev-qt/qtcore:5] Fix errors in cmake config file

The dev-qt/qtcore-5.2.0_beta1 ebuild previously installed a cmake configuration file (/usr/lib64/cmake/Qt5Core/Qt5CoreConfig.cmake) which caused cmake builds to fail with the following error (from emerge =dev-util/cmake-2.8.11.2):

CMake Error at /usr/lib64/cmake/Qt5Core/Qt5CoreConfig.cmake:116 (list):
  list sub-command REMOVE_DUPLICATES requires list to be present.
Call Stack (most recent call first):
  Tests/RunCMake/CMakeLists.txt:80 (find_package)

This patch alters the Qt5CoreConfig.cmake file to only REMOVE_DUPLICATES when the Qt5Core_PRIVATE_INCLUDE_DIRS variable is set & nonempty.