fixes a windows problem

the line causes my windows qt5.3.0 x64 to say:

CMake Error at source/propertyguizeug/CMakeLists.txt:8 (find_package):
  Found package configuration file:

    D:/Qt/Qt5.3.0/5.3/msvc2013_64_opengl/lib/cmake/Qt5/Qt5Config.cmake

  but it set Qt5_FOUND to FALSE so package "Qt5" is considered to be NOT
  FOUND.  Reason given by package:

  The Qt5 package requires at least one component