printk: Make vprintk_deferred() public

vprintk_deferred() is useful for implementing runtime verification
reactors. Make it public.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>