kmsg - kmsg_dump() fix CONFIG_PRINTK=n compilation

Signed-off-by: Kay Sievers <kay@vrfy.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>