Add <cassert> header include

This fixes some Linux build failures.

2021/puzzle-05-02.cc

@@ -2,6 +2,7 @@
 // Created by Matthew Gretton-Dann on 05/12/2021.
 //
 
+#include <cassert>
 #include <iostream>
 #include <map>
 #include <numeric>
@@ -69,4 +70,4 @@ auto main() -> int
 
   std::cout << "Danger spots: " << danger_count << '\n';
   return 0;
-}
+}