Update bindgen requirement from 0.46 to 0.47

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](https://github.com/rust-lang/rust-bindgen/commits/v0.47.0)

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

foundationdb-sys/Cargo.toml

@@ -30,4 +30,4 @@ fdb-6_0 = [ ]
 [dependencies]
 
 [build-dependencies]
-bindgen = "0.46"
+bindgen = "0.47"