Add <cassert> header include

This fixes some Linux build failures.

2016/puzzle-09-01.cc

@@ -2,6 +2,7 @@
 // Created by Matthew Gretton-Dann on 05/12/2021.
 //
 
+#include <cassert>
 #include <iostream>
 #include <string>
 
@@ -56,4 +57,4 @@ auto main() -> int
   auto output{decompress(input)};
   std::cout << "Output length: " << output.size() << '\n';
   return 0;
-}
+}