Add missing atomic include
This include was required to compile VTK with VTK_SMP_IMPLEMENTATION_TYPE=OpenMP
on my GCC 9.3.0 (GNU/Linux).
@seanm please review.
This include was required to compile VTK with VTK_SMP_IMPLEMENTATION_TYPE=OpenMP
on my GCC 9.3.0 (GNU/Linux).
@seanm please review.