Add <cassert> header include

This fixes some Linux build failures.

2015/puzzle-25-01.cc

@@ -1,3 +1,4 @@
+#include <cassert>
 #include <cstdint>
 #include <iostream>
 #include <string>
@@ -44,4 +45,4 @@ int main()
   std::cout << "Target row " << target_row << " column " << target_column << '\n';
   std::cout << "Row " << row << " column " << column << " num " << num << '\n';
   return 0;
-}
+}