Update bindgen requirement from 0.55 to 0.56 in /fitsio-sys-bindgen

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@v0.55.0...v0.56.0)

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

fitsio-sys-bindgen/Cargo.toml

@@ -21,7 +21,7 @@ tempfile = "3.1.0"
 
 [build-dependencies]
 pkg-config = "0"
-bindgen = "0.55"
+bindgen = "0.56"
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]