doc: install: add missing exiftool dependency

What does this MR do?

Adds the exiftool GitLab Workhorse dependency to the source installation documentation. The following comment mentions that the dependency is already described in https://gitlab.com/gitlab-org/gitlab-workhorse#dependencies. However, the official installation from source documentation does not mention this requirement, while it does state the following

This guide is long because it covers many cases and includes all commands you need ...

We use a source-based installation on our GitLab-EE instance and we encountered the problem in #28215 (closed) as we based our installation steps on the source documentation.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)