add jpeg to OpenBSD prereq list

[kivikakk]

Without the jpeg package installed, this occurs while installing the matrix-synapse Python package:

    The headers or library files could not be found for jpeg,
    a required dependency when compiling Pillow from source.

    Please see the install instructions at:
       https://pillow.readthedocs.io/en/latest/installation.html

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-eSNi9N/pillow/setup.py", line 814, in <module>
        raise RequiredDependencyException(msg)
    __main__.RequiredDependencyException:

    The headers or library files could not be found for jpeg,
    a required dependency when compiling Pillow from source.

    Please see the install instructions at:
       https://pillow.readthedocs.io/en/latest/installation.html

[kivikakk]

Guess who didn't read the contributing guidelines first? 🤦‍♀️ Retargetted develop, added sign-off.

[hawkowl]

@kivikakk Hi! One other thing you need is a changelog entry -- CONTRIBUTING has more details, but you want to make a file called <PR number>.bugfix containing details, so people know it works now.

Thanks!

[kivikakk]

@hawkowl Thanks for your patience! (And hello 😊) I've added the entry.