[PATCH] Constify proclist.h

Hi hackers,

This is a follow-up to [1] and c8ad4d81.

> Additionally Bharath pointed out that there are other pieces of code
> that we may want to change in a similar fashion,
> proclist.h/proclist_types.h as one example. I didn't do this yet
> because I would like to know the community opinion first on whether we
> should do this at all.

Since the consensus seems to be to constify everything possible here
is the patch for proclist.h. There is nothing to change in
proclist_types.h.

[1]: https://postgr.es/m/CAJ7c6TM2%3D08mNKD9aJg8vEY9hd%2BG4L7%2BNvh30UiNT3kShgRgNg%40mail.gmail.com

-- 
Best regards,
Aleksander Alekseev