Add back CStr

Author: mattusifer

src/config.rs

@@ -23,7 +23,7 @@
 //! [librdkafka-config]: https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md
 
 use std::collections::HashMap;
-use std::ffi::CString;
+use std::ffi::{CStr, CString};
 use std::iter::FromIterator;
 use std::os::raw::c_char;
 use std::ptr;