Bug#776191: Update licensing description in xxd.c

[Bram Moolenaar]

James -

> Hi Bram,
> 
> There was recently some discussion[0] about the licensing of xxd.  The
> result of which was Jürgen clarifying[1] that it should be dual-licensed
> under MIT-X11 and GPL-2 (or later).
> 
> [0]: https://lists.debian.org/debian-legal/2015/01/msg00033.html
> [1]: https://lists.debian.org/debian-legal/2015/01/msg00037.html
> 
> Would you mind updating the license description in xxd.c?

I'll add the text that Juergen mentioned.

- Bram

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