Update cbindgen requirement from 0.14 to 0.15

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@v0.14.0...v0.15.0)

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

ffi/Cargo.toml

@@ -27,4 +27,4 @@ default = []
 ffi-headers = ["cbindgen"]
 
 [build-dependencies]
-cbindgen = { version = "0.14", optional = true }
+cbindgen = { version = "0.15", optional = true }