[committed] Fix gdb.texinfo for old makeinfo

[Eli Zaretskii]

> Date: Thu, 11 Jan 2018 12:35:33 +0000
> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
> CC: nd@arm.com, John Baldwin <jhb@freebsd.org>, 
>  Eli Zaretskii <eliz@gnu.org>
> 
> With old makeinfo (version 4.13) the changes introduced in
> 
> commit 2d97a5d9d33aea87c3bd02fd1fa417f5d4e1fa05
> Author:     John Baldwin <jhb@FreeBSD.org>
> 
>     Document support for 'info proc' on FreeBSD.
> 
> fail to build.
> 
> (committed as obvious)

Thanks for catching this.