flatpak: Access to host cache for media art

Allow flatpaked app to use host cache media art. Would give a better
experience, since we don't allow goa on flatpak so it will never
download media art.

org.gnome.Music.json

@@ -15,7 +15,8 @@
         "--talk-name=ca.desrt.dconf", "--env=DCONF_USER_CONFIG_DIR=.config/dconf",
         "--talk-name=com.intel.dleyna-server",
         "--socket=pulseaudio",
-        "--filesystem=xdg-music"
+        "--filesystem=xdg-music",
+        "--filesystem=xdg-cache/media-art"
     ],
     "build-options" : {
         "cflags": "-O2 -g",