Avoid another warning about deprecated implicit copy ctor

This is similar to the previous commit, but in the other direction, with
the warning being generated for the implicitly-declared copy ctor due to
having an explicitly-declared assignment operator.

And the fix is different too because we need to have an assignment
operator in this test, so define the copy ctor explicitly too.

Examples/test-suite/private_assign.i

@@ -14,6 +14,8 @@
            return *this;
        }
    public:
+       Foo() { }
+       Foo(const Foo &f) { } // copy ctor can still be public, however.
        void bar() { }
    };