demo: Add libgudev into flatpak manifest

It is needed for SPI support.

demo/org.freedesktop.libfprint.Demo.json

@@ -47,6 +47,18 @@
                 }
             ]
         },
+        {
+            "name": "gudev",
+            "buildsystem": "meson",
+            "config-opts": [ "-Dtests=disabled", "-Dintrospection=disabled" ],
+            "sources": [
+                {
+                    "type": "archive",
+                    "url": "https://download.gnome.org/sources/libgudev/236/libgudev-236.tar.xz",
+                    "sha256": "e50369d06d594bae615eb7aeb787de304ebaad07a26d1043cef8e9c7ab7c9524"
+                }
+            ]
+        },
         {
             "name": "libfprint",
             "buildsystem": "meson",