Add libffi development packages to requirements

What does this merge request do and why?

As described in https://bugs.ruby-lang.org/issues/18571, Ruby 3.2 stopped bundling libffi to build the fiddle gem. As a result, fiddle is now an optional dependency if the libffi and its source headers are not present.

This causes Ruby interpreters that are built without the necessary dependencies to omit the fiddle C extension.

Make this a dependency in the GDK to ensure Ruby builds properly.

Relates to:

• gitlab#438901 (closed)
• #2035

How to set up and validate locally

You'd have to uninstall libffi-dev and reinstall ruby to see this error: gitlab#438901 (closed)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.