Update GCC 16 status for P1789

https://gcc.gnu.org/g:6159bfcf8b5852417519cf4181bf0ff8907ac0ca

Signed-off-by: Jonathan Wakely <cxx@kayari.org>

Author: jwakely

features_cpp26.yaml

@@ -1631,7 +1631,7 @@ features:
     summary: "Specializes `std::tuple_size`, `std::tuple_element`, and `std::get` for `std::integer_sequence` to make it usable in expansion statements and structured bindings."
     content: expansion-statements-library-support.md
     lib: true
-    support: [Clang 22 (hint)]
+    support: [Clang 22 (hint), GCC 16]
     ftm:
       - name: __cpp_lib_integer_sequence
         value: 202511L