This is really minor but the issue this fixes is that if you copy-paste this output of `--show-builtin` into your `build.zig` for example then the formatter will format
```
pub const os = std.Target.Os{
    .tag = .freestanding,
    .version_range = .{ .none = {} }
};
```
to
```
pub const os = std.Target.Os{ .tag = .freestanding, .version_range = .{ .none = {} } };
```
which doesn't match the output.
With this comma, the output will stay the way it is after a `zig fmt`.