Add <cassert> header include

This fixes some Linux build failures.

2016/puzzle-20-02.cc

@@ -1,3 +1,4 @@
+#include <cassert>
 #include <iomanip>
 #include <iostream>
 #include <numeric>
@@ -98,4 +99,4 @@ auto main() -> int
   std::cout << "First available IP address: " << inverse.lowest() << '\n';
   std::cout << "Number of available IP addresses: " << inverse.count() << '\n';
   return 0;
-}
+}