Add all missing deps for building flatpak

With this flatpak is now possible to build and working

pkgs/flatpak/lib/libass.json

@@ -0,0 +1,19 @@
+{
+    "name": "libass",
+    "config-opts": [ "--enable-shared", "--disable-static" ],
+    "sources": [
+        {
+            "type": "git",
+            "url": "https://github.com/libass/libass.git",
+            "tag": "0.14.0",
+            "commit": "73284b676b12b47e17af2ef1b430527299e10c17"
+        },
+        {
+            "type":"script",
+            "commands":[
+                "autoreconf -fiv"
+            ],
+            "dest-filename":"autogen.sh"
+        }
+    ]
+}