Fix build with pipewire 1.3.82

While submitting pipewire 1.3.82 to openSUSE Tumbleweed I noticed this
cast is needed to build xdg-desktop-portal. This was already submitted
as an issue in #1611, in which the reporter doesn't mention the
used pipewire version but says "I am also building my own pipewire"
so I guess it's the latest RC (1.3.81 or 1.3.82).

Fixes: #1611

src/pipewire.c

@@ -331,7 +331,7 @@ pipewire_remote_new_sync (struct pw_properties *pipewire_properties,
   remote->registry = (struct pw_proxy*) pw_core_get_registry (remote->core,
                                                               PW_VERSION_REGISTRY,
                                                               0);
-  pw_registry_add_listener (remote->registry,
+  pw_registry_add_listener ((struct pw_registry*)remote->registry,
                             &remote->registry_listener,
                             &registry_events,
                             remote);