Add <cassert> header include

This fixes some Linux build failures.

2021/puzzle-04-01.cc

@@ -3,6 +3,7 @@
 //
 
 #include <array>
+#include <cassert>
 #include <iomanip>
 #include <iostream>
 #include <numeric>
@@ -147,4 +148,4 @@ auto main() -> int
 
   std::cerr << "No bingo!\n";
   return 1;
-}
+}

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;
-}
+}

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;
-}
+}

2021/puzzle-08-02.cc

@@ -3,6 +3,7 @@
 //
 
 #include <array>
+#include <cassert>
 #include <iostream>
 #include <string>
 #include <vector>
@@ -219,4 +220,4 @@ auto main() -> int
 
   std::cout << "Total: " << total << '\n';
   return 0;
-}
+}