Fix Since tags

Fixes https://github.com/behdad/harfbuzz/issues/103

src/hb-buffer.h

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 1cfe13c..bb89dc3 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -171,6 +171,9 @@
 hb_buffer_guess_segment_properties (hb_buffer_t *buffer);
 
 
+/*
+ * Since: 0.9.20
+ */
 typedef enum { /*< flags >*/
   HB_BUFFER_FLAG_DEFAULT			= 0x00000000u,
   HB_BUFFER_FLAG_BOT				= 0x00000001u, /* Beginning-of-text */
@@ -185,6 +188,9 @@
 hb_buffer_flags_t
 hb_buffer_get_flags (hb_buffer_t *buffer);
 
+/*
+ * Since: 0.9.42
+ */
 typedef enum {
   HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES	= 0,
   HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS	= 1,
@@ -319,6 +325,9 @@
  * Serialize
  */
 
+/*
+ * Since: 0.9.20
+ */
 typedef enum { /*< flags >*/
   HB_BUFFER_SERIALIZE_FLAG_DEFAULT		= 0x00000000u,
   HB_BUFFER_SERIALIZE_FLAG_NO_CLUSTERS		= 0x00000001u,