crypto/asymmetric_keys: pkcs7_key_type needs module.h

This driver builds off of the tristate CONFIG_PKCS7_TEST_KEY and calls
module_init and module_exit. So it should explicitly include module.h
to avoid compile breakage during header shuffles done in the future.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>