[babl] HCY: add alpha meta data for model

[Øyvind "pippin" Kolås <ok src gnome org>]

commit f98c0737f27998db28d09b5d243f9420ffcddaee
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Nov 27 00:22:37 2018 +0100

    HCY: add alpha meta data for model

 extensions/HCY.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/extensions/HCY.c b/extensions/HCY.c
index e78d704..96063e2 100644
--- a/extensions/HCY.c
+++ b/extensions/HCY.c
@@ -94,6 +94,7 @@ models (void)
     babl_component ("HCY chroma"),
     babl_component ("HCY luma"),
     babl_component ("alpha"),
+    "alpha",
     NULL
   );