Add @generated to the output erl

The intention is to mitigate performance issues with
the Erlang laguage platform.

src/gpb_compile.erl

@@ -4392,7 +4392,8 @@ format_erl(Mod, Defs, DefsNoRenamings, DefsForIntrospect,
     DoDecoders = gpb_lib:get_gen_decoders(Opts),
     CompileOptsStr = get_erlc_compile_options_str(Opts),
     gpb_lib:iolist_to_utf8_or_escaped_binary(
-      [?f("%% @private~n"
+      [?f("%% % this file is @generated~n"
+          "%% @private~n"
           "%% Automatically generated, do not edit~n"
           "%% Generated by ~p version ~s~n"
           "%% Version source: ~s~n",