[forward.list.overview] Add missing constexpr (#8717)

Author: hewillk

source/containers.tex

@@ -7117,7 +7117,7 @@
     constexpr size_type remove(const T& value);
     template<class Predicate> constexpr size_type remove_if(Predicate pred);
 
-    size_type unique();
+    constexpr size_type unique();
     template<class BinaryPredicate> constexpr size_type unique(BinaryPredicate binary_pred);
 
     constexpr void merge(forward_list& x);