Fix problem with clean command in Makefile

Author: yhirose

example/Makefile

@@ -58,4 +58,4 @@ pem:
 	openssl req -new -key key.pem | openssl x509 -days 3650 -req -signkey key.pem > cert.pem
 
 clean:
-	rm server client hello simplecli simplesvr upload redirect ssesvr ssecli benchmark *.pem
+	rm server client hello simplecli simplesvr upload redirect ssesvr ssecli benchmark one_time_request *.pem