Fix the unit test of adapt_struct for consistency with other tests of the same kind.

test/sequence/adapt_struct.cpp

@@ -159,7 +159,7 @@ main()
     {
         // conversion from point to list
         point p = {5, 3, 3};
-        list<int, long> l(p);
+        list<int, long, int> l(p);
         l = p;
     }