meson: add subproject fallback for zstd

meson: add subproject fallback for zstd

installed via the command:

```
meson wrap install zstd
```

This file does nothing, unless a dependency cannot be found; in that
case, instead of failing, meson will download a private copy of the
dependency and build + link to that instead.

(This is especially common on Windows, where managing dependencies
without a package manager can be difficult.)

Alternatively, users interested in specifically building a statically
linked / portable executable can configure meson with
`--wrap-mode=forcefallback` to prefer the private copy.