Fix discarding const qualifier with OpenSSL master branch

OpenSSL's master branch is going to be OpenSSL 4.0. These functions
return const pointers:

  - PKCS7_get_signed_attribute()
  - X509_ATTRIBUTE_get0_object()
  - X509_ATTRIBUTE_get0_type()

Author: rhenium

ext/openssl/ossl_pkcs7.c

@@ -1010,7 +1010,7 @@ static VALUE
 ossl_pkcs7si_get_signed_time(VALUE self)
 {
     PKCS7_SIGNER_INFO *p7si;
-    ASN1_TYPE *asn1obj;
+    const ASN1_TYPE *asn1obj;
 
     GetPKCS7si(self, p7si);
 

ext/openssl/ossl_x509attr.c

@@ -196,7 +196,7 @@ ossl_x509attr_set_value(VALUE self, VALUE value)
         ossl_raise(eX509AttrError, "attribute value must be ASN1::Set");
 
     if (X509_ATTRIBUTE_count(attr)) { /* populated, reset first */
-        ASN1_OBJECT *obj = X509_ATTRIBUTE_get0_object(attr);
+        const ASN1_OBJECT *obj = X509_ATTRIBUTE_get0_object(attr);
         X509_ATTRIBUTE *new_attr = X509_ATTRIBUTE_create_by_OBJ(NULL, obj, 0, NULL, -1);
         if (!new_attr) {
             sk_ASN1_TYPE_pop_free(sk, ASN1_TYPE_free);
@@ -240,7 +240,7 @@ ossl_x509attr_get_value(VALUE self)
 
     count = X509_ATTRIBUTE_count(attr);
     for (i = 0; i < count; i++)
-        sk_ASN1_TYPE_push(sk, X509_ATTRIBUTE_get0_type(attr, i));
+        sk_ASN1_TYPE_push(sk, (ASN1_TYPE *)X509_ATTRIBUTE_get0_type(attr, i));
 
     if ((len = i2d_ASN1_SET_ANY(sk, NULL)) <= 0) {
         sk_ASN1_TYPE_free(sk);