Upgrade rust/crates/nix to 0.21.0

Test: make
Change-Id: Ic8422bfdc4afc11c6c310e67999ec360d92d843d
diff --git a/src/sys/statvfs.rs b/src/sys/statvfs.rs
index 9bea973..508fa8d 100644
--- a/src/sys/statvfs.rs
+++ b/src/sys/statvfs.rs
@@ -1,6 +1,6 @@
 //! Get filesystem statistics
 //!
-//! See [the man pages](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fstatvfs.html)
+//! See [the man pages](https://pubs.opengroup.org/onlinepubs/9699919799/functions/fstatvfs.html)
 //! for more details.
 use std::mem;
 use std::os::unix::io::AsRawFd;
@@ -54,7 +54,7 @@
 
 /// Wrapper around the POSIX `statvfs` struct
 ///
-/// For more information see the [`statvfs(3)` man pages](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_statvfs.h.html).
+/// For more information see the [`statvfs(3)` man pages](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_statvfs.h.html).
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct Statvfs(libc::statvfs);