sim: remove deprecated usage of sync::ONCE_INIT

Switch to `Once::new()`.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/src/testlog.rs b/sim/src/testlog.rs
index 28a189a..8d3d267 100644
--- a/sim/src/testlog.rs
+++ b/sim/src/testlog.rs
@@ -7,9 +7,9 @@
 //! the tests.
 
 use env_logger;
-use std::sync::{Once, ONCE_INIT};
+use std::sync::Once;
 
-static INIT: Once = ONCE_INIT;
+static INIT: Once = Once::new();
 
 /// Setup the logging system.  Intended to be called at the beginning of each test.
 pub fn setup() {