printk: add dummy routine for when CONFIG_PRINTK=n

There are missing dummy routines for log_buf_addr_get() and
log_buf_len_get() for when CONFIG_PRINTK is not set causing build
failures.

This patch adds these dummy routines at the appropriate location.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Petr Mladek <pmladek@suse.cz>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>