Add <cassert> header include

This fixes some Linux build failures.

2016/puzzle-01-01.cc

@@ -2,6 +2,7 @@
 // Created by Matthew Gretton-Dann on 02/12/2021.
 //
 
+#include <cassert>
 #include <iostream>
 #include <string>
 
@@ -66,4 +67,4 @@ auto main() -> int
   vert = std::abs(vert);
   std::cout << "We end up " << horiz << " + " << vert << " = " << horiz + vert << " blocks away.\n";
   return 0;
-}
+}