added header guard

Author: Xin Dong

src/algorithms/reco/Helix.h

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright (C) 2024 Xin Dong, Rongrong Ma
 
+#pragma once
+
 #include <algorithm>
 #include <cmath>
 #include <exception>