Fix usage of PKCS#7 ASN1 attribute retrieval for SCEP keygen
Since pre-1.0.0 code should not have used ASN1_ITEM references
directly like &X509_it. It should instead use the macro
This worked up to 3.0.0.
Switch the code to more current usage of ASN1_ITEM and referencing
the PKCS7 signer info object.
Signed-off-by: Rob Crittenden <firstname.lastname@example.org>