Add __weak sync_file_range()

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/helpers.c b/helpers.c
index 9b1f3e9..779f9fb 100644
--- a/helpers.c
+++ b/helpers.c
@@ -29,3 +29,9 @@
 
 	return ret;
 }
+
+int __weak sync_file_range(int fd, off64_t offset, off64_t nbytes,
+			   unsigned int flags)
+{
+	return 0;
+}