backend/headless: add support for disabled outputs

We have nothing to do when a headless output is enabled/disabled.

backend/headless/output.c

@@ -9,6 +9,7 @@
 static const uint32_t SUPPORTED_OUTPUT_STATE =
 	WLR_OUTPUT_STATE_BACKEND_OPTIONAL |
 	WLR_OUTPUT_STATE_BUFFER |
+	WLR_OUTPUT_STATE_ENABLED |
 	WLR_OUTPUT_STATE_MODE;
 
 static size_t last_output_num = 0;