Add Qt private headers to CMake project

If you want to use the private API of the "QTMOD" submodule then you must include only the headers using the variable:

Qt${QT_VERSION_MAJOR}QTMOD_INCLUDE_DIRS

In your case:

include_directories(${Qt${QT_VERSION_MAJOR}Quick_PRIVATE_INCLUDE_DIRS})

_{Note: Not exist QuickPrivate}