[gdb/contrib] Support gdb in codespell section of setup.cfg

Add support for the gdb dir in the codespell section of gdb/contrib/setup.cfg,
specifically adding files in the skip line.

This allows us to run codespell from the command line on the gdb dir:
...
$ codespell --config gdb/contrib/setup.cfg gdb 2>/dev/null | wc -l
1665
...
without running into warnings in generated files.

Approved-By: Tom Tromey <tom@tromey.com>