Added essential missing export declaration

The VTK_ABI_EXPORT is required specially for modules outside VTK or else
the essential python init function becomes hidden causing a error something
like this: importError: dynamic module does not define init function

Change-Id: I125f9521becd19e330a810ce82468b6b27d0c067
Merge topic 'fix_python_wrapping_external' into master

5cdc2ece Added essential missing export declaration