zstd: Fix non-FreeBSD CI libzstd build

Fix non-FreeBSD CI build after v1.4.8. This definition was only used in
zstd(1), which isn't part of non-FreeBSD CI (I guess). The ifdef was
added in v1.4.5 import.

Upstream does not currently support shared-linked zstd(1), but I have
proposed https://github.com/facebook/zstd/pull/2450 . If that is
adopted, we can add -DZSTD_PROGRAMS_LINK_SHARED to our libzstd build and
drop some diffs.

Reported by: uqs