chore(schmea): include the emqx_exhook_schema

apps/emqx_machine/src/emqx_machine_schema.erl

@@ -54,6 +54,7 @@
         , emqx_dashboard_schema
         , emqx_gateway_schema
         , emqx_prometheus_schema
+        , emqx_exhook_schema
         ]).
 
 %% TODO: add a test case to ensure the list elements are unique