[rust][edition] Specify Rust 2018 Edition

Change-Id: I9e464efd4e74cf212bce179f7fe7e1fca392f1db
diff --git a/rust/boringssl-sys/BUILD.gn b/rust/boringssl-sys/BUILD.gn
index ed9db61..b951c0d 100644
--- a/rust/boringssl-sys/BUILD.gn
+++ b/rust/boringssl-sys/BUILD.gn
@@ -8,6 +8,7 @@
   name = "boringssl-sys"
   version = "0.1.0"
   with_unit_tests = true
+  edition = "2018"
 
   non_rust_deps = [
     # only depend on libcrypto until we actually use the SSL bits