[babl] ycbcr: add model alpha meta data

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

commit 36c5a254e6c3a5cf5bbe172f1815f3e024a1b1a4
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Nov 27 00:24:25 2018 +0100

    ycbcr: add model alpha meta data

 extensions/ycbcr.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/extensions/ycbcr.c b/extensions/ycbcr.c
index 200aa78..a71577b 100644
--- a/extensions/ycbcr.c
+++ b/extensions/ycbcr.c
@@ -67,6 +67,7 @@ models (void)
     babl_component ("Cb"),
     babl_component ("Cr"),
     babl_component ("alpha"),
+    "alpha",
     NULL);
 }