flatpak-run-wayland: Change security-context sandbox engine name

To align with work in D-Bus, the security-context's sandbox engine names
are now supposed to follow the reverse-DNS style.

This change should be fine because there has not been a release of
flatpak with security-context support, yet.

common/flatpak-run-wayland.c

@@ -137,7 +137,7 @@ flatpak_run_add_wayland_security_context_args (FlatpakBwrap *bwrap,
  security_context = wp_security_context_manager_v1_create_listener (security_context_manager,
  listen_fd,
  sync_fd);
- wp_security_context_v1_set_sandbox_engine (security_context, "flatpak");
+ wp_security_context_v1_set_sandbox_engine (security_context, "org.flatpak");
  wp_security_context_v1_set_app_id (security_context, app_id);
  wp_security_context_v1_set_instance_id (security_context, instance_id);
  wp_security_context_v1_commit (security_context);