[committed] Fix gdb.texinfo for old makeinfo (again)

[Szabolcs Nagy]

With old makeinfo (version 4.13) the changes introduced in

commit ba643918cf869fa0d064d733f69b453b6fe642ea
Author:     Sergio Durigan Junior <sergiodj@redhat.com>

    Install and generate docs for gdb-add-index

fail to build with

gdb/doc/gdb.texinfo:2498: warning: `.' or `,' must follow @xref, not `@'.
gdb/doc/gdb.texinfo:2517: warning: `.' or `,' must follow @xref, not `@'.
gdb/doc/gdb.texinfo:43443: Node `gdb-add-index man' requires a sectioning command (e.g., @unnumberedsubsec).
gdb/doc/gdb.texinfo:43443: `gdb-add-index man' has no Up field (perhaps incorrect sectioning?).
gdb/doc/gdb.texinfo:43350: Next field of node `gdbinit man' not pointed to (perhaps incorrect sectioning?).
gdb/doc/gdb.texinfo:43443: This node (gdb-add-index man) has the bad Prev.

This patch fixes the warnings too.
Committed as obvious.

gdb/doc/ChangeLog:
2018-01-15  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* gdb.texinfo (set cwd): Add period.
	(gdb-add-index): Move anchor.