bio/bss_file.c: since VS2015 one can't tell apart own and "alien" FILE

pointers, except for minimal std[in|out|err].

(cherry picked from commit 800b299b

)
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2701)