hide internal csrf session key helper

src/lib/uri.cpp

@@ -877,7 +877,7 @@ void session_destroy(String session_name)
 	}
 }
 
-String csrf_session_key(String token_name)
+static String csrf_session_key(String token_name)
 {
 	token_name = first(trim(token_name), "csrf_token");
 	return("_csrf_" + token_name);