build: add flatpak manifest to testing in Builder

I give it session-bus access so that we can connect to a Qemu instance that
was started on the user session. Not needed for "real" apps.

org.gnome.libmks.mks.json

@@ -0,0 +1,37 @@
+{
+    "app-id" : "org.gnome.libmks.mks",
+    "runtime" : "org.gnome.Platform",
+    "runtime-version" : "master",
+    "sdk" : "org.gnome.Sdk",
+    "command" : "mks",
+    "finish-args" : [
+        "--socket=fallback-x11",
+        "--device=dri",
+        "--socket=wayland",
+        "--socket=session-bus"
+    ],
+    "cleanup" : [
+        "/include",
+        "/lib/pkgconfig",
+        "/share/gir-1.0",
+        "/share/vala",
+        "*.la",
+        "*.a"
+    ],
+    "modules" : [
+        {
+            "name" : "mks",
+            "builddir" : true,
+            "buildsystem" : "meson",
+            "config-opts" : [
+                "-Dinstall-tools=true"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://gitlab.gnome.org/chergert/libmks.git"
+                }
+            ]
+        }
+    ]
+}