Fix: add missing header <functional> to gsl/pointers due to using les…

…s/greater

include/gsl/pointers

@@ -20,6 +20,7 @@
 #include "assert" // for Ensures, Expects
 
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
+#include <functional>   // for less, greater
 #include <memory>       // for shared_ptr, unique_ptr, hash
 #include <type_traits>  // for enable_if_t, is_convertible, is_assignable
 #include <utility>      // for declval, forward