fix(cluster-webui): export emqx_modules_api:list_modules/1

Fix http.500 error when loading page:http://127.0.0.1:18083/#/modules on
a emqx cluster,

lib-ce/emqx_modules/src/emqx_modules_api.erl

@@ -67,6 +67,7 @@
             descr  => "Reload a module in the cluster"}).
 
 -export([ list/2
+        , list_modules/1
         , load/2
         , unload/2
         , reload/2