chore: use iwyu on sorting/**.cpp

sorting/shell_sort2.cpp

@@ -3,12 +3,11 @@
  * \brief [Shell sort](https://en.wikipedia.org/wiki/Shell_sort) algorithm
  * \author [Krishna Vedala](https://github.com/kvedala)
  */
-#include <cassert>
-#include <cstdlib>
-#include <ctime>
-#include <iostream>
-#include <utility>  // for std::swap
-#include <vector>
+#include <cassert>   // for assert
+#include <cstdlib>   // for rand, size_t, qsort, srand, NULL, atoi
+#include <ctime>     // for clock, clock_t, CLOCKS_PER_SEC, time
+#include <iostream>  // for operator<<, basic_ostream, char_traits, cout, endl
+#include <vector>    // for vector
 
 /** pretty print array
  * \param[in] arr array to print