Stub out the OpenSSL secure heap.

OpenSSL added a separate "secure heap" to allocate some data in a
different heap. We don't implement this, so just act as if initializing
it always fails. Node now expects these functions to be available.

Change-Id: I4c57c807c51681b16ec3a60e9674583b193358c4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/54309
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index 9abccaf..3a721fe 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -967,6 +967,9 @@
 // conservative.)
 #define BN_prime_checks BN_prime_checks_for_validation
 
+// BN_secure_new calls |BN_new|.
+OPENSSL_EXPORT BIGNUM *BN_secure_new(void);
+
 
 // Private functions