OpenSSL: Fix the same BIO_FLAGS macro definition

Also add comment to the public header to avoid
making another conflict in future.

Reviewed by: jkim
Obtained from: OpenSSL commit 5d4975ecd88ac17d0749513a8fac9a7c7befd900
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34135