Fixed build error on stable due to FFI namespace changes.

Fortune for mapped-file's current commit: Future small blessing − 末小吉

src/err.rs

@@ -1,6 +1,6 @@
 //! Wrapping `errno` error types
 use super::*;
-use std::ffi::c_int;
+use std::os::raw::c_int;
 use std::{
     fmt, error
 };