Fix warning about deprecated implicit conversion.

[marcfehling]

This should take care of the warning introduced in #15415 (comment).

[kronbichler]

There is one additional error, now at the linker stage (showing that the error in #15415 was indeed not just spurious):

 /usr/bin/ld: ../lib/libdeal_II.g.so.9.6.0-pre: undefined reference to `void dealii::internal::hermite_hierarchic_to_lexicographic_numbering<0>(unsigned int, std::vector<unsigned int, std::allocator<unsigned int> >&)'
/usr/bin/ld: ../lib/libdeal_II.g.so.9.6.0-pre: undefined reference to `dealii::FiniteElementData<0>::FiniteElementData(std::vector<unsigned int, std::allocator<unsigned int> > const&, unsigned int, unsigned int, dealii::FiniteElementData<0>::Conformity, dealii::BlockIndices const&)'

[peterrum]

I think the undefined references should be fixed now. There is some recursive template calling which is actually terminates with dim=1.