cred: #include init.h in cred.h

linux/cred.h can't be included as first header (alphabetical order)
because it uses __init which is enough to break compilation on some archs.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>