[gnome-build-meta] gdm.bst: Update for GDM master→main branch rename

[Ray Strode <halfline src gnome org>]

commit 3e0677a5e57d969380c21b51a9147873645bf651
Author: Ray Strode <halfline gmail com>
Date:   Wed Sep 29 19:19:56 2021 +0000

    gdm.bst: Update for GDM master→main branch rename
    
    GDM now uses main instead of master for its main branch.

 elements/core/gdm.bst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/elements/core/gdm.bst b/elements/core/gdm.bst
index e33413200..e22af4c03 100644
--- a/elements/core/gdm.bst
+++ b/elements/core/gdm.bst
@@ -2,7 +2,7 @@ kind: meson
 sources:
 - kind: git_tag
   url: gitlab_gnome_org:GNOME/gdm.git
-  track: master
+  track: main
 - kind: local
   path: files/gdm
   directory: gdm-config