bfd: fix generation of bfd.texi in out-of-tree builds

[Jose E. Marchesi]

> On Thu, Jan 23, 2025 at 08:57:45PM +0100, Jose E. Marchesi wrote:
>> Since currently TS/bfd/doc/bfdt.texi is outdated and is missing some
>> nodes, the error above happens.
>
> Um, TS/bfd/doc/bfdt.text doesn't exist in my source tree.

Oh I thought it was in git, but it isn't.
Mine is from May 10 2023 :)

>
>> This patch changes bfd/doc/local.mk to use -P to prepend the current
>
> Sounds a good idea, please apply.

Pushed, thanks.