cli: static keyword tweak

cli: static keyword tweak

Function ubus_cli_listen() is called only once, and function uloop_run()
and uloop_done() which need struct ubus_event_handler listener are
called within this function, so it's not needed to mark struct
ubus_event_handler listener as static. The same as pointer struct
ubus_context *ctx in main function.

Global struct commands is only used in this file, so it should be marked
static.

Signed-off-by: Zhao, Gang <gang.zhao.42@gmail.com>