kmsg_dump.h: fix build when CONFIG_PRINTK is disabled

Fix <linux/kmsg_dump.h> when CONFIG_PRINTK is not enabled:

  include/linux/kmsg_dump.h:56: error: 'EINVAL' undeclared (first use in this function)
  include/linux/kmsg_dump.h:61: error: 'EINVAL' undeclared (first use in this function)

Looks like commit 595dd3d8bf ("kmsg_dump: fix build for
CONFIG_PRINTK=n") uses EINVAL without having the needed header file(s),
but I'm sure that I build tested that patch also.  oh well.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kmsg_dump.h

@@ -12,6 +12,7 @@
 #ifndef _LINUX_KMSG_DUMP_H
 #define _LINUX_KMSG_DUMP_H
 
+#include <linux/errno.h>
 #include <linux/list.h>
 
 enum kmsg_dump_reason {