Add header guard in builtin.h.

include/builtin.h

@@ -1,3 +1,5 @@
+#ifndef BUILTIN_H
+#define BUILTIN_H
 #include <cmath>
 #include "object.h"
 
@@ -114,3 +116,4 @@ const ObjectBase* Arifm1(const ObjectList* input)
 
  return new ObjectError("Arifmetic unary operator","unknown error"); // Impossible case
 }
+#endif