Added ${PROJECT_SOURCE_DIR}/include to the list of pkg-config include directories for catkin packages

The include directory in the source directory of the package was not exported for catkin packages until now. The exported include directory `${CATKIN_DEVEL_PREFIX}/include/orocos` will only contain generated headers, like for typekits.

So once #5 is solved, we probably will need this - or additionally rely on the catkin-way of exporting the include directories via `catkin_package(INCLUDE_DIRS ...)`. Thoughts?