fix(xref): Ignore xref for generated modules for protobuf

rebar.config

@@ -57,5 +57,7 @@
   {emqx_schema_api,make_schema_params,1},
   {emqx_schema_parser,decode,3},
   {emqx_schema_parser,encode,3},
-  {emqx_schema_registry,add_schema,1}
+  {emqx_schema_registry,add_schema,1},
+  emqx_exhook_pb, % generated code for protobuf
+  emqx_exproto_pb % generated code for protobuf
 ]}.