chunker - fix 4GB files on 32-bit systems

From code inspection - effect not actually tested.

borg/_chunker.c

@@ -83,7 +83,8 @@ typedef struct {
     PyObject *fd;
     int fh;
     int done, eof;
-    size_t remaining, bytes_read, bytes_yielded, position, last;
+    size_t remaining, position, last;
+    off_t bytes_read, bytes_yielded;
 } Chunker;
 
 static Chunker *