BUG: Duplicate class definition of vtkSeedCallback2

Previous commit(7d5b24444) introduced a new issue of multiple classes with
same name(vtkSeedCallback2) in  TestSeedWidget2 and
TestSeedWidgetNonUniformRepresentation. This change fixes the issue.

Change-Id: Ie7a7274971c4779acd666cf395e992f287eb705f
Merge topic 'fix_TestSeedWidget_failure' into master

c458abe7 BUG: Duplicate class definition of vtkSeedCallback2