Fix dialyzering for OTP 27

[kivra-pauoli]

Motivation

We have an internal dependency that uses this lib. and it's failing dialyzer analysis.

Further considerations

{dialyzer, [{plt_extra_apps, [xmerl, erlsom]}]}. is required because otherwise the applications aren't loaded for analysis (if e.g. they were in .app.src this wouldn't be required).

[kivra-pauoli]

I don't know of a better way to do this.

1. the erlsom:simple_form() type doesn't exist
2. in our tests, for example, the return is a tuple, but without any proper "form"

My goal in adding the comment is to have the consumer look at the dependency, if they need it for that specific type.