Fix typo.

Change-Id: I4720fdf0f943b9f196a57ceec340b4161f04ea5d
Reviewed-on: https://boringssl-review.googlesource.com/18304
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: Robert Sloan <varomodt@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/bn.h

diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index 0d2068f..bdd41ba 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -938,7 +938,7 @@
 #if defined(__cplusplus)
 }  /* extern C */
 
-#if !defined(OPENSSL_NO_CXX)
+#if !defined(BORINGSSL_NO_CXX)
 extern "C++" {
 
 namespace bssl {