fix: add an include for functional in catalan_numbers

Author: realstealthninja

dynamic_programming/catalan_numbers.cpp

@@ -9,11 +9,12 @@
     https://oeis.org/A000108/
  */
 
-#include <cassert>  /// for assert
-#include <cstdint>  /// for std::uint64_t
-#include <cstdlib>  /// for std::size_t
-#include <numeric>  /// for std::transform_reduce
-#include <vector>   /// for std::vector
+#include <cassert>     /// for assert
+#include <cstdint>     /// for std::uint64_t
+#include <cstdlib>     /// for std::size_t
+#include <functional>  /// for std::plus
+#include <numeric>     /// for std::transform_reduce
+#include <vector>      /// for std::vector
 
 /**
  * @brief computes and caches Catalan numbers