[llvm] fix zstd_shared detection on mingw (PR #139945)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM.

CCing @petrhosek who I think authored this cmake file, in case he has opinions on it. Otherwise I can probably merge it in a day or two (or whenever I remember it again).

https://github.com/llvm/llvm-project/pull/139945