chore: use iwyu on data_structures/**.cpp

data_structures/queue_using_two_stacks.cpp

@@ -5,9 +5,10 @@
  * Implementation of a Queue using two Stacks.
  */
 
-#include <cassert>
-#include <iostream>
-#include <stack>
+#include <stdlib.h>  // for exit
+#include <cassert>   // for assert
+#include <iostream>  // for operator<<, basic_ostream, char_traits, cout, cerr
+#include <stack>     // for stack
 
 namespace {
 /**