Add prerequisite #include directives to include/crypto/x509.h

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11808)

include/crypto/x509.h

diff --git a/include/crypto/x509.h b/include/crypto/x509.h
index c663b68..a3c05f1 100644
--- a/include/crypto/x509.h
+++ b/include/crypto/x509.h
@@ -8,6 +8,8 @@
  */
 
 #include "internal/refcount.h"
+#include <openssl/asn1.h>
+#include <openssl/x509.h>
 
 /* Internal X509 structures and functions: not for application use */