Update cbindgen requirement from 0.17 to 0.18 (#26)

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@v0.17.0...v0.18.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

ffi/Cargo.toml

@@ -27,4 +27,4 @@ default = []
 ffi-headers = ["cbindgen"]
 
 [build-dependencies]
-cbindgen = { version = "0.17", optional = true }
+cbindgen = { version = "0.18", optional = true }