Fix class detection macro

Weird whitespace confused the regex. The old regex failed to catch:

    class FOO_EXPORTS  vtkFoo : public vtkObject

because of the double space.

Change-Id: Ie64477a7a49a35a0b7f14f06343dea7ad440a720
Merge topic 'update-class-detection-regex' into master

4bfb80cb Fix class detection macro