Add <cassert> header include

This fixes some Linux build failures.

2021/puzzle-04-02.cc

@@ -3,6 +3,7 @@
 //
 
 #include <array>
+#include <cassert>
 #include <iomanip>
 #include <iostream>
 #include <numeric>
@@ -151,4 +152,4 @@ auto main() -> int
   std::cout << "\nBINGO " << score << " * " << *num << " = " << score * *num << '\n';
   cards[0].print(std::cout);
   return 0;
-}
+}