Remove unused ${SOURCE_DIR}/include include path

The directory `${CMAKE_CURRENT_SOURCE_DIR}/include` is never created or used. Except in case of an in-source build but then  `${CMAKE_CURRENT_BINARY_DIR}/include` would be the same, so it would only duplicate the correct entry.

Not sure if the intermediary `JANSSON__INCLUDE_DIRS` is still necessary but I thought I'd change as little as possible.

The reason for eliminating this is that when you use CLion and sync into a docker container it will try to copy all include dirs back. Non-existant paths cause warnings every time.