nfsd: v4 support requires CRYPTO

nfsd V4 support uses crypto interfaces, so select CRYPTO
to fix build errors in 2.6.39:

ERROR: "crypto_destroy_tfm" [fs/nfsd/nfsd.ko] undefined!
ERROR: "crypto_alloc_base" [fs/nfsd/nfsd.ko] undefined!

Reported-by: Wakko Warner <wakko@animx.eu.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>