backend: Fix build with -Dremote_desktop=false
../mutter-44.0/src/backends/meta-backend.c: In function ‘meta_backend_real_post_init’:
../mutter-44.0/src/backends/meta-backend.c:560:7: error: ‘MetaBackendPrivate’ {aka ‘struct _MetaBackendPrivate’} has no member named ‘remote_access_controller’
560 | priv->remote_access_controller =
| ^~