Merge pull request #2234 from sfackler/alex-patch-1

Only declare OpensslCallbacks in bindgen builds

Author: alex

openssl-sys/build/run_bindgen.rs

@@ -205,6 +205,7 @@ pub fn run_boringssl(include_dirs: &[PathBuf]) {
         .compile("boring_static_wrapper");
 }
 
+#[cfg(feature = "bindgen")]
 #[derive(Debug)]
 struct OpensslCallbacks;