make X509_REQ opaque

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/crypto/x509/x509_r2x.c b/crypto/x509/x509_r2x.c
index d9c3cfd..abf75cd 100644
--- a/crypto/x509/x509_r2x.c
+++ b/crypto/x509/x509_r2x.c
@@ -62,6 +62,7 @@
 #include <openssl/evp.h>
 #include <openssl/asn1.h>
 #include <openssl/x509.h>
+#include "internal/x509_int.h"
 #include <openssl/objects.h>
 #include <openssl/buffer.h>