pgtk copyright headers

[Po Lu]

The files in the PGTK port have nonsensical dates in their copyright
headers.

Before update-copyright is run tomorrow (happy new year, by the way), I
think it would be appropriate to fix those dates:

  - pgtkterm.c, pgtkfns.c, and pgtkim.c say they are copyright 1989 to
    2020, while (judging by the VCS history) they should say 2017-2021.

  - pgtkgui.h and pgtkterm.h say they are copyright 2008-2020; they
    should also say 2017-2021.

  - pgtkmenu.c says it is copyright 2019-2020, while it should say
    2019-2021 instead.

  - pgtkselect.c says it is copyright 1993 to 2020, while it should say
    2017-2021.

Is it okay for me to make the necessary corrections now, or are there
some legal issues that have to be taken care of first?

Thanks in advance.