Add missing pragma once

This header didn't include a pragma once.  Fix it.

Tested: Code compiles

Change-Id: I8bd4f9d870ec9b7dc1687e8de1c8a61d93140c7e
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/include/json_html_serializer.hpp b/include/json_html_serializer.hpp
index ede2c89..a4bcdcb 100644
--- a/include/json_html_serializer.hpp
+++ b/include/json_html_serializer.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 #include "http_response.hpp"
 
 #include <nlohmann/json.hpp>