use off_t instead of size_t for file size

stat() st_size datatype is off_t which is for file size, use off_t
instead of size_t for correctness.

src/core.h

@@ -40,7 +40,7 @@ struct tip_client {
 	/* for file serving. */
 	const char		*uri;
 	const char		*path;
-	size_t			size;
+	off_t			size;
 	int			fd;
 	off_t			offset;