Add pbgen command-line to comment in generated file

[cgay]

Add the pbgen command-line to the comment in the generated file that currently looks like this:

// *** This file was auto-generated by pbgen. ***
//     Source: /home/cgay/dylan/workspaces/pb/protocol-buffers/test-data/descriptor.proto
//     Date: 2024-01-21T22:03:40+00:00