Update bindgen requirement from 0.55.1 to 0.56.0

Updates the requirements on [bindgen](https://github.com/rust-lang/rust-bindgen) to permit the latest version.
- [Release notes](https://github.com/rust-lang/rust-bindgen/releases)
- [Changelog](https://github.com/rust-lang/rust-bindgen/blob/master/CHANGELOG.md)
- [Commits](rust-lang/rust-bindgen@v0.55.1...v0.56.0)

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

Author: dependabot-preview[bot]

foundationdb-sys/Cargo.toml

@@ -39,4 +39,4 @@ fdb-6_2 = []
 [dependencies]
 
 [build-dependencies]
-bindgen = "0.55.1"
+bindgen = "0.56.0"