o_fopen.c: compensate for e_os.h omission.

At earlier point e_os.h was omitted from a number of headers (in order
to emphasize OS neutrality), but this affected o_fopen.c, which is not
OS-neutral, and contains some DJGPP-specific code.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5708)