config/output: don't leak background_fallback

(cherry picked from commit 30c8584)

Author: llyyr

Diff for: sway/config/output.c

@@ -1018,6 +1018,7 @@ void free_output_config(struct output_config *oc) {
 	free(oc->name);
 	free(oc->background);
 	free(oc->background_option);
+	free(oc->background_fallback);
 	wlr_color_transform_unref(oc->color_transform);
 	free(oc);
 }