add copyright header (facebookresearch#3948)

Summary:
Pull Request resolved: facebookresearch#3948

I noticed this file I added was violating the header check. https://www.internalfb.com/intern/opensource/github/repo/1812028399049977/checkup/

Reviewed By: asadoughi

Differential Revision: D64341054

fbshipit-source-id: 4564191661acbff193c8ffe970582cef8fb3a490

faiss/python/faiss_example_external_module.swig

@@ -1,3 +1,11 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// This is an example of how an external module can be added via SWIG.
 
 %module faiss_example_external_module;