cmd: add missing usage vars

we should have caught them all - hopefully.

Change-Id: I35435317fccaf5ad0216244d69f76db6857bb582
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/381
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/server/server.c

@@ -625,6 +625,7 @@ static const struct command_registration server_command_handlers[] = {
 		.name = "shutdown",
 		.handler = &handle_shutdown_command,
 		.mode = COMMAND_ANY,
+		.usage = "",
 		.help = "shut the server down",
 	},
 	COMMAND_REGISTRATION_DONE