Add missing imports in ui tests

There is a new fallback as of nightly-2018-12-29 that makes these emit a
new error unrelated to Serde.

test_suite/tests/ui/conflict/flatten-tuple-struct.rs

@@ -1,6 +1,8 @@
 #[macro_use]
 extern crate serde_derive;
 
+use std::collections::HashMap;
+
 #[derive(Serialize)]
 struct Foo(u32, #[serde(flatten)] HashMap<String, String>);