Include bn.h from bn/internal.h

If using precompiled headers then this is needed otherwise bn/internal.h
doesn't have a definition for BN_ULONG etc.

Change-Id: I41b331465abae7108f255722a156d2ffb3016ba3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44604
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/crypto/fipsmodule/bn/internal.h b/crypto/fipsmodule/bn/internal.h
index c3aa21b..e11ee45 100644
--- a/crypto/fipsmodule/bn/internal.h
+++ b/crypto/fipsmodule/bn/internal.h
@@ -123,7 +123,7 @@
 #ifndef OPENSSL_HEADER_BN_INTERNAL_H
 #define OPENSSL_HEADER_BN_INTERNAL_H
 
-#include <openssl/base.h>
+#include <openssl/bn.h>
 
 #if defined(OPENSSL_X86_64) && defined(_MSC_VER)
 OPENSSL_MSVC_PRAGMA(warning(push, 3))