ui/dbus: fix clang compilation issue

../ui/dbus-listener.c:236:9: error: expected expression
        Error *err = NULL;

See:
https://gitlab.com/qemu-project/qemu/-/issues/1782#note_1488517427

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230726151221.515761-1-marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

ui/dbus-listener.c

@@ -232,7 +232,7 @@ static void dbus_call_update_gl(DisplayChangeListener *dcl,
         egl_fb_read_rect(ddl->ds, &ddl->fb, x, y, w, h);
         dbus_gfx_update(dcl, x, y, w, h);
         break;
-    case SHARE_KIND_D3DTEX:
+    case SHARE_KIND_D3DTEX: {
         Error *err = NULL;
         assert(ddl->d3d_texture);
 
@@ -249,6 +249,7 @@ static void dbus_call_update_gl(DisplayChangeListener *dcl,
             dbus_update_gl_cb,
             g_object_ref(ddl));
         break;
+    }
     default:
         g_warn_if_reached();
     }