flatpak: Add appstream-glib module

It's no longer part of the SDK, so we have to build it ourselves

org.gnome.Geary.json

@@ -269,6 +269,24 @@
                 }
             ]
         },
+        {
+            "name": "appstream-glib",
+            "buildsystem": "meson",
+            "config-opts": [
+                "-Dfonts=false",
+                "-Dgtk-doc=false",
+                "-Dintrospection=false",
+                "-Dman=false",
+                "-Drpm=false"
+            ],
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://github.com/hughsie/appstream-glib.git",
+                    "branch": "main"
+                }
+            ]
+        },
         {
             "name": "geary",
             "buildsystem": "meson",