Constify some X509_CRL functions.

Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/x509/x509cset.c

@@ -95,22 +95,22 @@ int X509_CRL_up_ref(X509_CRL *crl)
     return ((i > 1) ? 1 : 0);
 }
 
-long X509_CRL_get_version(X509_CRL *crl)
+long X509_CRL_get_version(const X509_CRL *crl)
 {
     return ASN1_INTEGER_get(crl->crl.version);
 }
 
-ASN1_TIME *X509_CRL_get_lastUpdate(X509_CRL *crl)
+ASN1_TIME *X509_CRL_get_lastUpdate(const X509_CRL *crl)
 {
     return crl->crl.lastUpdate;
 }
 
-ASN1_TIME *X509_CRL_get_nextUpdate(X509_CRL *crl)
+ASN1_TIME *X509_CRL_get_nextUpdate(const X509_CRL *crl)
 {
     return crl->crl.nextUpdate;
 }
 
-X509_NAME *X509_CRL_get_issuer(X509_CRL *crl)
+X509_NAME *X509_CRL_get_issuer(const X509_CRL *crl)
 {
     return crl->crl.issuer;
 }