fix: declare `emqx` as a dependency in `.app.src`

apps/emqx_message_transformation/src/emqx_message_transformation.app.src

@@ -5,7 +5,8 @@
     {mod, {emqx_message_transformation_app, []}},
     {applications, [
         kernel,
-        stdlib
+        stdlib,
+        emqx
     ]},
     {env, []},
     {modules, []},