Use an anonymous namespace for vtkImageMedian3DAccumulateMedian

This places the vtkImageMedian3DAccumulateMedian() function in an
anonymous namespace rather than using static to give is file-scope.

This fixes the following compilation error with the IBM xlC compiler:
"The name lookup for "vtkImageMedian3DAccumulateMedian" did not find
a declaration.".

Change-Id: I795ec861950d4c042925f51444af80984c618431
Merge topic 'fix-xlc-compilation-error' into master

bd049ad5 Use an anonymous namespace for vtkImageMedian3DAccumulateMedian