Update cbindgen requirement from 0.19 to 0.20 (#30)

Updates the requirements on [cbindgen](https://github.com/eqrion/cbindgen) to permit the latest version.
- [Release notes](https://github.com/eqrion/cbindgen/releases)
- [Changelog](https://github.com/eqrion/cbindgen/blob/master/CHANGES)
- [Commits](mozilla/cbindgen@v0.19.0...v0.20.0)

---
updated-dependencies:
- dependency-name: cbindgen
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

ffi/Cargo.toml

@@ -27,4 +27,4 @@ default = []
 ffi-headers = ["cbindgen"]
 
 [build-dependencies]
-cbindgen = { version = "0.19", optional = true }
+cbindgen = { version = "0.20", optional = true }