crypto: ahash - unexport crypto_ahash_type

Now that all the templates that need ahash spawns have been converted to
use crypto_grab_ahash() rather than look up the algorithm directly,
crypto_ahash_type is no longer used outside of ahash.c.  Make it static.

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