One more f->file_name typo

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 5bf8c43..c5764c8 100644
--- a/fio.h
+++ b/fio.h
@@ -238,7 +238,7 @@
 		unsigned long file_data;
 		int fd;
 	};
-	const char *file_name;
+	char *file_name;
 	void *mmap;
 	unsigned long long file_size;
 	unsigned long long real_file_size;