[gnome-calculator] flatpak: Use the gtksourceview-4-8 branch

[Robert Roth <robertroth src gnome org>]

commit 0b8cbe6e8247b6319abebcc5e264808a5aa69446
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Thu Nov 5 10:37:31 2020 +0100

    flatpak: Use the gtksourceview-4-8 branch
    
    The master branch of gtksourceview has version 5.0, and we need 4.8 to
    build.

 org.gnome.Calculator.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/org.gnome.Calculator.json b/org.gnome.Calculator.json
index 1372c51d..c4227b7f 100644
--- a/org.gnome.Calculator.json
+++ b/org.gnome.Calculator.json
@@ -62,7 +62,8 @@
             "sources" : [
                 {
                     "type" : "git",
-                    "url" : "https://gitlab.gnome.org/GNOME/gtksourceview.git";
+                    "url" : "https://gitlab.gnome.org/GNOME/gtksourceview.git";,
+                    "branch" : "gtksourceview-4-8"
                 }
             ]
         },