Add all missing deps for building flatpak

With this flatpak is now possible to build and working

pkgs/flatpak/gstreamer-1.0/gstreamer.json

@@ -0,0 +1,24 @@
+{
+    "name": "gstreamer",
+    "buildsystem": "meson",
+    "config-opts": [
+        "-Ddoc=disabled",
+        "-Dgtk_doc=disabled",
+        "-Dexamples=disabled",
+        "-Dtests=disabled",
+        "-Dbenchmarks=disabled",
+        "-Dnls=disabled",
+        "-Dgobject-cast-checks=disabled",
+        "-Dglib-asserts=disabled",
+        "-Dglib-checks=disabled",
+        "-Dextra-checks=disabled"
+    ],
+    "sources": [
+        {
+            "type": "git",
+            "url": "https://gitlab.freedesktop.org/gstreamer/gstreamer.git",
+            "tag": "1.18.1",
+            "commit": "29a8099d1d4bd8717c13923e710e92e67e335353"
+        }
+    ]
+}