crypto: user - forward declare crypto_nlsk

Move the declaration of crypto_nlsk into internal/cryptouser.h.  This
fixes the following sparse warning:

    crypto/crypto_user_base.c:41:13: warning: symbol 'crypto_nlsk' was not declared. Should it be static?

Cc: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>