Fix Since tags

Fixes https://github.com/behdad/harfbuzz/issues/103
diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index a9664ee..13ba5d9 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -125,6 +125,9 @@
   CFRelease (data);
 }
 
+/*
+ * Since: 0.9.10
+ */
 CGFontRef
 hb_coretext_face_get_cg_font (hb_face_t *face)
 {