Update cbindgen requirement from 0.24 to 0.26

Updates the requirements on [cbindgen](https://github.com/mozilla/cbindgen) to permit the latest version.
- [Release notes](https://github.com/mozilla/cbindgen/releases)
- [Changelog](https://github.com/mozilla/cbindgen/blob/master/CHANGES)
- [Commits](mozilla/cbindgen@v0.24.0...0.26.0)

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

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

ffi/Cargo.toml

@@ -28,4 +28,4 @@ default = []
 ffi-headers = ["cbindgen"]
 
 [build-dependencies]
-cbindgen = { version = "0.24", optional = true }
+cbindgen = { version = "0.26", optional = true }