Update add_python_export_library.cmake

${CATKIN_PACKAGE_PYTHON_DESTINATION} is e.g. "lib/python2.7/dist-packages/adtf_streaming" for package adtf_streaming. This is already the correct target destination.
Update add_python_export_library.cmake

This works for me
Merge pull request #90 from uschwes/patch-2

Correct install targets for add_python_export_library macro