Hide the Python wrapper AddFile functions

It is not necessary to export these functions, since they are only
called from the Init.cxx file within the same module.  Hiding them
results in a tidier DLL interface.

Wrapping/Tools/vtkWrapPython.c

@@ -395,7 +395,7 @@ int VTK_PARSE_MAIN(int argc, char* argv[])
 
   /* do the export of the main entry point */
   fprintf(
-    fp, "extern \"C\" { %s void PyVTKAddFile_%s(PyObject *dict); }\n", "VTK_ABI_EXPORT", name);
+    fp, "extern \"C\" { %s void PyVTKAddFile_%s(PyObject *dict); }\n", "VTK_ABI_HIDDEN", name);
 
   /* get the module that is being wrapped */
   data = file_info->MainClass;