opentitan-uart: Re-add the edition

Apparently this codebase does require the 2018 edition flag in the Cargo.toml
for some reason. Let's fix this first, then figure out why.

Change-Id: I60699f77377a2cfee170228a55095d2813d96715

chips/opentitan/Cargo.toml

diff --git a/chips/opentitan/Cargo.toml b/chips/opentitan/Cargo.toml
index a7ef8ba..b7a0004 100644
--- a/chips/opentitan/Cargo.toml
+++ b/chips/opentitan/Cargo.toml
@@ -2,6 +2,7 @@
 name = "opentitan"
 version = "0.1.0"
 authors = ["Tock Project Developers <tock-dev@googlegroups.com>"]
+edition = "2018"
 
 [dependencies]
 rv32i = { path = "../../../tock/arch/rv32i" }