Specify the "runtime" feature of bindgen.

As it became non-default in rust-lang/rust-bindgen#1620.

Author: emilio

lmdb-sys/Cargo.toml

@@ -28,7 +28,7 @@ libc = "0.2"
 [build-dependencies]
 pkg-config = "0.3"
 cc = "1.0"
-bindgen = { version = "0.53.2", default-features = false, optional = true }
+bindgen = { version = "0.53.2", default-features = false, optional = true, features = ["runtime"] }
 
 [features]
 default = []