build(deps): update cbindgen requirement in /bindings/c

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@0.27.0...0.29.0)

---
updated-dependencies:
- dependency-name: cbindgen
  dependency-version: 0.29.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

bindings/c/Cargo.toml

@@ -12,7 +12,7 @@ crate-type = ["cdylib"]
 libc = "0.2"
 
 [build-dependencies]
-cbindgen = "0.27"
+cbindgen = "0.29"
 
 [dependencies.css-inline]
 path = "../../css-inline"