add include guard for all headers

include/soi

@@ -1,14 +1,11 @@
 // -*- c++ -*-
-/*
-   Students: please don't try to understand the details of headers just
-   yet.  All will be explained.  This header is primarily used so that you don't
-   have to understand every concept all at once.
-*/
 //          Copyright Johannes Kapfhammer 2019-2022.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          http://www.boost.org/LICENSE_1_0.txt)
 
+#ifndef SOI_HEADER_INCLUDED
+#define SOI_HEADER_INCLUDED
 
 #include "bits/include-all.hpp"
 
@@ -93,3 +90,5 @@ using soi::interactive_task;
 
 #define int int64_t
 using namespace std;
+
+#endif // SOI_HEADER_INCLUDED