Upgrade rust/crates/nix to 0.21.0

Test: make
Change-Id: Ic8422bfdc4afc11c6c310e67999ec360d92d843d
diff --git a/test/test_poll.rs b/test/test_poll.rs
index a5e2d25..acfaad8 100644
--- a/test/test_poll.rs
+++ b/test/test_poll.rs
@@ -11,7 +11,7 @@
             match $poll_expr {
                 Ok(nfds) => break nfds,
                 Err(Error::Sys(Errno::EINTR)) => (),
-                Err(e) => panic!(e)
+                Err(e) => panic!("{}", e)
             }
         }
     }