Update rust-bingen to 0.51

SimonSapin · SimonSapin · commit 99c7a16bca68 · 2019-09-24T13:59:25.000+02:00
… to pick up rust-lang/rust-bindgen#1627
diff --git a/Cargo.toml b/Cargo.toml
@@ -31,6 +31,6 @@ libc = "0.2"
 libz-sys = "1.0"
 
 [build-dependencies]
-bindgen = "0.49.0"
+bindgen = "0.51.1"
 cc = "1.0"
-walkdir = "2"
+walkdir = "2"
