[epiphany/mcatanzaro/libportal-update] Update flatpak manifest for latest libportal

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 6c74771080b69096b9d4692a1508497955469602
Author: Michael Catanzaro <mcatanzaro redhat com>
Date:   Tue Nov 9 13:42:08 2021 -0600

    Update flatpak manifest for latest libportal
    
    Fixes #1621

 org.gnome.Epiphany.json | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/org.gnome.Epiphany.json b/org.gnome.Epiphany.json
index 77993c115..77522a14b 100644
--- a/org.gnome.Epiphany.json
+++ b/org.gnome.Epiphany.json
@@ -103,6 +103,7 @@
             "name" : "libportal",
             "buildsystem" : "meson",
             "config-opts" : [
+                "-Dbackends=gtk3",
                 "-Dgtk_doc=false"
             ],
             "sources" : [