[babl] HSL: add alpha model meta-data

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

commit 6d21209309b1bbe600acebf24e37cb23714eeee3
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Nov 27 00:23:59 2018 +0100

    HSL: add alpha model meta-data

 extensions/HSL.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/extensions/HSL.c b/extensions/HSL.c
index 76d263b..20cb8b2 100644
--- a/extensions/HSL.c
+++ b/extensions/HSL.c
@@ -62,6 +62,7 @@ init (void)
                   babl_component ("saturation"),
                   babl_component ("lightness"),
                   babl_component ("alpha"),
+                  "alpha",
                   NULL);
   babl_model_new ("name", "HSL",
                   babl_component ("hue"),