[llvm] [CMake] Fix config when static zstd libraries are not found (PR #113584)

[Aaron Puchert via llvm-commits]

https://github.com/aaronpuchert commented:

If you emit an error, it seems like you can leave `LLVM_ENABLE_ZSTD` as it is.

https://github.com/llvm/llvm-project/pull/113584