Fix copyright statement check

Do not check for the copyright statement for files in the third party
directory. These files shouldn't have the VTK-m copyright. Frankly, I
don't understand why this has not been a problem before.

LICENSE.txt

@@ -77,6 +77,7 @@ vtkm/cont/tbb/internal/parallel_sort.h
 vtkm/cont/tbb/internal/parallel_radix_sort_tbb.h
 vtkm/cont/tbb/internal/kxsort.h
 vtkm/testing/OptionParser.h
+vtkm/thirdparty
 vtkm/internal/brigand.hpp
 version.txt