Update sip_helper to use python header dirs

This fixes https://github.com/ros-visualization/qt_gui_core/pull/46 and https://github.com/ros-visualization/rviz/pull/788; needed at least on OS X, likely some other platforms as well.
Can you please add the folllowing line before this:

    set(Python_ADDITIONAL_VERSIONS "${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR}")

It ensures that find_package() is able to find the specified Python version - even if the version number is unknown in the PythonLibs CMake config file.