Flatpak: add builddir: true to the manifests

GNOME Builder carries a reimplementation of flatpak-builder.
flatpak-builder assumes that builddir is true for meson, but
Builder does not since it also supports out-of-tre builds.

This helps indicated to Builder that builddir can indeed be used.

org.gnome.Podcasts.json

@@ -62,6 +62,7 @@
         },
         {
             "name" : "gnome-podcasts",
+            "builddir" : "true",
             "buildsystem" : "meson",
             "sources" : [
                 {

org.gnome.PodcastsDevel.json

@@ -63,6 +63,7 @@
         {
             "name" : "gnome-podcasts",
             "buildsystem" : "meson",
+            "builddir" : "true",
             "config-opts" : [ "-Dprofile=development" ],
             "sources" : [
                 {