Fix warning on HTG 2D FullMaterial test.

Change-Id: Ia3f85f0d0ccf18993dbbdd63ede5ec4fa963da79

Filters/HyperTree/Testing/Cxx/TestHyperTreeGridTernary2DFullMaterialBits.cxx

@@ -165,7 +165,7 @@ int TestHyperTreeGridTernary2DFullMaterialBits( int argc, char* argv[] )
   idArray->SetNumberOfComponents( 1 );
   vtkIdType nbPoints = htGrid->GetOutput()->GetNumberOfPoints();
   idArray->SetNumberOfValues( nbPoints );
-  for ( unsigned int i = 0; i < nbPoints; ++ i )
+  for ( vtkIdType i = 0; i < nbPoints; ++ i )
     {
     idArray->SetValue( i, i );
     }