nfsctl: add headers for credentials

Needs headers help for current_cred:

Adding only cred.h wasn't enough.

linux-next-20081023/fs/nfsctl.c:45: error: implicit declaration of function 'current_cred'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>