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-newtype-struct.rs

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

test_suite/tests/ui/conflict/flatten-newtype-struct.stderr

@@ -1,7 +1,7 @@
 error: #[serde(flatten)] cannot be used on newtype structs
- --> $DIR/flatten-newtype-struct.rs:5:12
+ --> $DIR/flatten-newtype-struct.rs:7:12
   |
-5 | struct Foo(#[serde(flatten)] HashMap<String, String>);
+7 | struct Foo(#[serde(flatten)] HashMap<String, String>);
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error

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>);
 

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

@@ -1,7 +1,7 @@
 error: #[serde(flatten)] cannot be used on tuple structs
- --> $DIR/flatten-tuple-struct.rs:5:17
+ --> $DIR/flatten-tuple-struct.rs:7:17
   |
-5 | struct Foo(u32, #[serde(flatten)] HashMap<String, String>);
+7 | struct Foo(u32, #[serde(flatten)] HashMap<String, String>);
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error