Make sure the ->files array is job private

Otherwise we introduce differences between threads and processes,
we don't want to do that.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index d0629ec..8a79289 100644
--- a/fio.h
+++ b/fio.h
@@ -708,6 +708,7 @@
 extern void put_file(struct thread_data *, struct fio_file *);
 extern int add_dir_files(struct thread_data *, const char *);
 extern int init_random_map(struct thread_data *);
+extern void dup_files(struct thread_data *, struct thread_data *);
 
 /*
  * ETA/status stuff