math/vtk8: unbreak aarch64 on FreeBSD 13

Builds fine locally in poudriere on 13.3 jail on aarch64.

I think it compiles now because FreeBSD 13.3 has a newer clang/llvm
version than FreeBSD 13.2.

PR: 264834
Approved by: yuri@ (maintainer) (similar patch as for vtk9)