Add missing test_metadata.h

Without test_metadata.h, when running with REGENERATE_METADATA=OFF,
cmake deletes test_metadata.c and then fails with file not found
error.

Signed-off-by: Jiri Kucera <jkucera@redhat.com>

cpp/src/phonenumbers/test_metadata.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2011 The Libphonenumber 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
+ *
+ * http://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.
+ */
+
+#ifndef I18N_PHONENUMBERS_TEST_METADATA_H_
+#define I18N_PHONENUMBERS_TEST_METADATA_H_
+
+namespace i18n {
+namespace phonenumbers {
+
+int metadata_size();
+const void* metadata_get();
+
+}  // namespace phonenumbers
+}  // namespace i18n
+
+#endif  // I18N_PHONENUMBERS_TEST_METADATA_H_