Add all missing deps for building flatpak

With this flatpak is now possible to build and working

pkgs/flatpak/gstreamer-1.0/gst-libav.json

@@ -0,0 +1,20 @@
+{
+    "name": "gst-libav",
+    "buildsystem": "meson",
+    "config-opts": [
+        "-Ddoc=disabled",
+        "-Dtests=disabled"
+    ],
+    "sources": [
+        {
+            "type": "git",
+            "url": "https://gitlab.freedesktop.org/gstreamer/gst-libav.git",
+            "tag": "1.18.1",
+            "commit": "097313530cae4a49437a779a9ded0ade8113c26b"
+        },
+        {
+            "type": "patch",
+            "path": "gst-libav-stop-caching-codecs.patch"
+        }
+    ]
+}