Make file.h C++ safe by casting fio_file_flags

Make file.h C++ safe by casting fio_file_flags

Fixes for g++ (4.7.2) following compiler errors when fio.h
gets included (e.g. in an external C++ ioengine):

--8<---
[...]
os/../file.h: In function ‘void fio_file_set_open(fio_file*)’:
os/../file.h:142:1: error: invalid conversion from ‘int’ to ‘fio_file_flags’ [-fpermissive]
os/../file.h: In function ‘void fio_file_clear_open(fio_file*)’:
os/../file.h:142:1: error: invalid conversion from ‘int’ to ‘fio_file_flags’ [-fpermissive]
os/../file.h: In function ‘void fio_file_set_closing(fio_file*)’:
[...]
--->8---

Signed-off-by: Daniel Gollub <d.gollub@telekom.de>
Signed-off-by: Jens Axboe <axboe@fb.com>