rand/randfile.c: remove _XOPEN_SOURCE definition.

Defintions of macros similar to _XOPEN_SOURCE belong in command line
or in worst case prior first #include directive in source. As for
macros is was allegedly controlling. One can argue that we are
probably better off demanding S_IS* macros but there are systems
that just don't comply, hence this compromise solution...

Reviewed-by: Rich Salz <rsalz@openssl.org>