gdbserver: add missing --disable-packet options to help text

The help text for the --disable-packet option was missing one of the
possible values.

As this option is for maintainers only it is explicitly not documented
in gdb/doc/gdb.texinfo, so no update is needed there.

gdbserver/ChangeLog:

	* server.cc (gdbserver_usage): Add missing option to usage text.
	(gdbserver_show_disableable): Likewise.