kdebug.h: forward-declare struct struct notifier_block

alpha:

In file included from kernel/notifier.c:1:
include/linux/kdebug.h:14: warning: 'struct notifier_block' declared inside parameter list
include/linux/kdebug.h:14: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/kdebug.h:15: warning: 'struct notifier_block' declared inside parameter list
kernel/notifier.c:529: error: conflicting types for 'register_die_notifier'
include/linux/kdebug.h:14: error: previous declaration of 'register_die_notifier' was here
kernel/notifier.c:533: error: conflicting types for 'register_die_notifier'
include/linux/kdebug.h:14: error: previous declaration of 'register_die_notifier' was here
kernel/notifier.c:536: error: conflicting types for 'unregister_die_notifier'
include/linux/kdebug.h:15: error: previous declaration of 'unregister_die_notifier' was here
kernel/notifier.c:539: error: conflicting types for 'unregister_die_notifier'
include/linux/kdebug.h:15: error: previous declaration of 'unregister_die_notifier' was here

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>