[PATCH] Fix ref in gdb.texinfo

[Szabolcs Nagy]

Since

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

gdb/doc/ChangeLog:
2018-06-11  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* gdb.texinfo (maint check libthread-db): Add anchor.
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