Fix build with opaque OCSP_BASICRESP in LibreSSL 3.5.
The bug this works around should be fixed since LibreSSL 2.4.2
as far as I can tell.

Index: ext/openssl/ossl_ocsp.c
--- ext/openssl/ossl_ocsp.c.orig
+++ ext/openssl/ossl_ocsp.c
@@ -1093,7 +1093,7 @@ ossl_ocspbres_verify(int argc, VALUE *argv, VALUE self
      * exists in LibreSSL 2.1.10, 2.2.9, 2.3.6, 2.4.1.
      */
     if (!(flg & (OCSP_NOCHAIN | OCSP_NOVERIFY)) &&
-	sk_X509_num(x509s) && sk_X509_num(bs->certs)) {
+	sk_X509_num(x509s) && sk_X509_num(OCSP_resp_get0_certs(bs))) {
 	int i;
 
 	bs = ASN1_item_dup(ASN1_ITEM_rptr(OCSP_BASICRESP), bs);
