[gdb/contrib] Add missing copyright notices

I found a few files in gdb/contrib that were missing a copyright notice.

In codespell-ignore-words.txt, it was trivial to add, but I found a
codespell issue stating that it's not explicitly support, so I added a note
about that.

In codespell-dictionary.txt, I had to resort to a hack, using an "empty word"
rewrite rule:
...
-># Comment here.
...

Also, I found that empty lines are not allowed in codespell-dictionary.txt.  I
filed an issue about these two problems [1].

Since these two files were added in 2025, I've used 2025-2026 as copyright
years.

Tested by running "pre-commit run --all-files".

[1] https://github.com/codespell-project/codespell/issues/3901