Fix CI failure

https://travis-ci.org/nix-rust/nix/jobs/557674308
error[E0433]: failed to resolve. Use of undeclared type or module `std`
  --> test/test_fcntl.rs:36:5
   |
36 |     std::fs::File::create(&old_path).unwrap();
   |     ^^^ Use of undeclared type or module `std`

This works locally (with Rust 1.36.0), and in hindsight I'm a little
confused why. I thought you needed the 2018 edition to not need ::
before std in this context. In any case, easy enough to fix.

test/test_fcntl.rs

@@ -4,6 +4,7 @@ use nix::fcntl::{openat, open, OFlag, readlink, readlinkat, rename, renameat};
 use nix::sys::stat::Mode;
 use nix::unistd::{close, read};
 use tempfile::{self, NamedTempFile};
+use std::fs::File;
 use std::io::prelude::*;
 use std::os::unix::fs;
 
@@ -33,7 +34,7 @@ fn test_openat() {
 fn test_rename() {
     let old_dir = tempfile::tempdir().unwrap();
     let old_path = old_dir.path().join("old");
-    std::fs::File::create(&old_path).unwrap();
+    File::create(&old_path).unwrap();
     let new_dir = tempfile::tempdir().unwrap();
     let new_path = new_dir.path().join("new");
     rename(&old_path, &new_path).unwrap();
@@ -44,7 +45,7 @@ fn test_rename() {
 fn test_renameat() {
     let old_dir = tempfile::tempdir().unwrap();
     let old_dirfd = open(old_dir.path(), OFlag::empty(), Mode::empty()).unwrap();
-    std::fs::File::create(&old_dir.path().join("old")).unwrap();
+    File::create(&old_dir.path().join("old")).unwrap();
     let new_dir = tempfile::tempdir().unwrap();
     let new_dirfd = open(new_dir.path(), OFlag::empty(), Mode::empty()).unwrap();
     renameat(old_dirfd, "old", new_dirfd, "new").unwrap();