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/gdb_server.c

@@ -2603,6 +2603,7 @@ static const struct command_registration gdb_command_handlers[] = {
 		.help = "next stepi will return immediately allowing "
 			"GDB to fetch register state without affecting "
 			"target state",
+		.usage = ""
 	},
 	{
 		.name = "gdb_port",