[PATCH] Fix ref in gdb.texinfo

[Eli Zaretskii]

> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Date: Mon, 11 Jun 2018 18:15:12 +0100
> 
> commit 5045b3d78903d1f6faa4cb511fa801e50dfacf48
> Commit:     Gary Benson <gbenson@redhat.com>
> CommitDate: 2018-06-08 18:06:46 +0100
> 
>      linux: Add maintenance commands to test libthread_db
> 
> The build fails for me with
> 
> ../../../gdb/doc/gdb.texinfo:35875: @ref reference to nonexistent node `maint check libthread-db'
> Makefile:486: recipe for target 'gdb.info' failed

Please try again, I hope I fixed that.