asn1.py: treat asn1_string_st as opaque

OpenSSL 4 plans to make ASN1_STRING opaque [1]. Using a forward
declaration rather than a redefinition avoids the build breakage
in cryptography's cffi when it tries to validate the sizes of the
members.

[1]: openssl/openssl#29117

Author: botovq

src/_cffi_src/openssl/asn1.py

@@ -13,12 +13,7 @@
 
 typedef ... ASN1_INTEGER;
 
-struct asn1_string_st {
-    int length;
-    int type;
-    unsigned char *data;
-    long flags;
-};
+struct asn1_string_st;
 
 typedef struct asn1_string_st ASN1_OCTET_STRING;
 typedef struct asn1_string_st ASN1_IA5STRING;