[sushi] flatpak: update for meson port

[Cosimo Cecchi <cosimoc src gnome org>]

commit 85e859167963f25a755560456eb08f7b838c18a1
Author: Cosimo Cecchi <cosimoc gnome org>
Date:   Mon Sep 3 20:53:37 2018 -0700

    flatpak: update for meson port

 flatpak/org.gnome.NautilusPreviewer.json | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/flatpak/org.gnome.NautilusPreviewer.json b/flatpak/org.gnome.NautilusPreviewer.json
index d507465..4565b82 100644
--- a/flatpak/org.gnome.NautilusPreviewer.json
+++ b/flatpak/org.gnome.NautilusPreviewer.json
@@ -109,6 +109,7 @@
         },
         {
             "name": "sushi",
+            "buildsystem": "meson",
             "sources": [
                 {
                     "type": "git",