compositor: fix leak in load_drm_backend()

Leak found running drm-smoke-test with ASan.

Do not forget to free string specific_device in load_drm_backend().

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

compositor/main.c

@@ -2674,6 +2674,7 @@ load_drm_backend(struct weston_compositor *c,
 
 	free(config.gbm_format);
 	free(config.seat_id);
+	free(config.specific_device);
 
 	return ret;
 }