[libcxx][iterator] adds `std::ranges::next`

Implements part of P0896 'The One Ranges Proposal'.
Implements [range.iter.op.next].

Depends on D101922.

Differential Revision: https://reviews.llvm.org/D102563

NOKEYCHECK=True
GitOrigin-RevId: 857fa7b7b1872a4a9b4072b6e83c85524c7229af
diff --git a/include/iterator b/include/iterator
index e490210..a094049 100644
--- a/include/iterator
+++ b/include/iterator
@@ -488,6 +488,7 @@
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iter_move.h>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
 #include <__iterator/readable_traits.h>
 #include <__memory/addressof.h>
 #include <__memory/pointer_traits.h>