test: fix 'incompatible pointer types' warnings

Refs: libuv#2686
PR-URL: libuv#3088
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

Author: cjihrig

test/test-poll-multiple-handles.c

@@ -83,10 +83,10 @@ TEST_IMPL(poll_multiple_handles) {
   /* Closing an already stopped polling handle is safe in any case */
   uv_close((uv_handle_t*) &first_poll_handle, close_cb);
 
-  uv_unref(&second_poll_handle);
+  uv_unref((uv_handle_t*) &second_poll_handle);
   ASSERT(0 == uv_run(uv_default_loop(), UV_RUN_DEFAULT));
   ASSERT(close_cb_called == 1);
-  uv_ref(&second_poll_handle);
+  uv_ref((uv_handle_t*) &second_poll_handle);
 
   ASSERT(uv_is_active((uv_handle_t*) &second_poll_handle));
   uv_close((uv_handle_t*) &second_poll_handle, close_cb);