libwevserv: Forgot one more virtual function in the Response base class

AddHeader must also be virtual to let MockResponse override it.

Change-Id: Id01ca3624a8f65703cab6106954a6395ba28a57b
diff --git a/libwebserv/response.h b/libwebserv/response.h
index c890e72..b0408c4 100644
--- a/libwebserv/response.h
+++ b/libwebserv/response.h
@@ -40,8 +40,8 @@
   virtual ~Response() = default;
 
   // Adds a single HTTP response header to the response.
-  void AddHeader(const std::string& header_name,
-                 const std::string& value);
+  virtual void AddHeader(const std::string& header_name,
+                         const std::string& value);
 
   // Adds number of HTTP response headers to the response.
   virtual void AddHeaders(