[gnome-builder] build: only build gtk3 libportal backend

[Christian Hergert <chergert src gnome org>]

commit 9b2a89ab341c11b9405a62b04935e24952cd1c14
Author: Christian Hergert <chergert redhat com>
Date:   Mon Nov 8 12:07:47 2021 -0800

    build: only build gtk3 libportal backend
    
    We don't really need them anyway, but especially not anything other than
    GTK 3 for Builder which is GTK 3.

 build-aux/flatpak/org.gnome.Builder.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 07f6fb5fb..e0e8f0df7 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -598,7 +598,8 @@
             "name" : "libportal",
             "config-opts" : [
                 "--libdir=/app/lib",
-                "--buildtype=debugoptimized"
+                "--buildtype=debugoptimized",
+                "-Dbackends=gtk3"
             ],
             "buildsystem" : "meson",
             "builddir" : true,