Add util.h as header for util.c.

Previously we were abusing primary_with_secondary.h.

Change-Id: I07b91a4e96d1c27bee4f5250724f119c126f6770
diff --git a/test/vmapi/primary_with_secondaries/inc/util.h b/test/vmapi/primary_with_secondaries/inc/util.h
new file mode 100644
index 0000000..b127fae
--- /dev/null
+++ b/test/vmapi/primary_with_secondaries/inc/util.h
@@ -0,0 +1,24 @@
+/*
+ * Copyright 2018 The Hafnium Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+struct mailbox_buffers {
+	void *send;
+	void *recv;
+};
+
+struct mailbox_buffers set_up_mailbox(void);