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

[Alan Modra]

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 Since currently TS/bfd/doc/bfdt.texi is outdated and is missing some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.

> This patch changes bfd/doc/local.mk to use -P to prepend the current

Sounds a good idea, please apply.

-- 
Alan Modra