sim: mcuboot-sys: 2018 edition fix

Automatic migration to Rust 2018:

    cargo fix --edition

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/src/lib.rs b/sim/mcuboot-sys/src/lib.rs
index 9f5e6b5..9f5c850 100644
--- a/sim/mcuboot-sys/src/lib.rs
+++ b/sim/mcuboot-sys/src/lib.rs
@@ -10,4 +10,4 @@
 // functions are exported to C code.
 pub mod api;
 
-pub use area::{AreaDesc, FlashId};
+pub use crate::area::{AreaDesc, FlashId};