file: Only include unistd.h if it exists

It is included for the open/read/write/close functions. On
MSVC, where this header does not exist, the same functions
are provided by io.h, which is already included.

On windows, these functions are provided by io.h. Make sure
io.h is included if it exists, regardless of the setmode
function.

Signed-off-by: Martin Storsjö <martin@martin.st>