[bindings] Fix hb_buffer_get_segment_properties

Annotate the output parameter.
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index eecb9d0..0543073 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -999,7 +999,7 @@
 /**
  * hb_buffer_get_segment_properties:
  * @buffer: a buffer.
- * @props: 
+ * @props: (out):
  *
  * 
  *