Restore the X509 ASN1_ITEM

https://boringssl-review.googlesource.com/c/boringssl/+/63946 removed
it, but sadly wpa_supplicant depends on it.

Change-Id: Ib3aca5269d740457ba83ca529b797bfb4a089763
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/64907
Reviewed-by: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
diff --git a/include/openssl/x509.h b/include/openssl/x509.h
index b27c97d..6fb07aa 100644
--- a/include/openssl/x509.h
+++ b/include/openssl/x509.h
@@ -123,6 +123,10 @@
 
 DEFINE_STACK_OF(X509)
 
+// X509 is an |ASN1_ITEM| whose ASN.1 type is X.509 Certificate (RFC 5280) and C
+// type is |X509*|.
+DECLARE_ASN1_ITEM(X509)
+
 // X509_up_ref adds one to the reference count of |x509| and returns one.
 OPENSSL_EXPORT int X509_up_ref(X509 *x509);