Fix compilation with CONFIG_MEMORY_HOTPLUG=y and gcc41.
Also remove unneeded declations, add a public function.

drivers/base/memory.c:53: error: static declaration of 'register_memory_notifier' follows non-static declaration
include/linux/memory.h:85: error: previous declaration of 'register_memory_notifier' was here
drivers/base/memory.c:58: error: static declaration of 'unregister_memory_notifier' follows non-static declaration
include/linux/memory.h:86: error: previous declaration of 'unregister_memory_notifier' was here
drivers/base/memory.c:68: error: static declaration of 'register_memory' follows non-static declaration
include/linux/memory.h:73: error: previous declaration of 'register_memory' was here

Signed-off-by: Olaf Hering <olh@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>