[gnome-continuous] Disable CUPS print backend for gtk4

[Emmanuele Bassi <ebassi src gnome org>]

commit 7027dadd0f36c8b72dc348aa5fb6e8d2ae909fb2
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sun Mar 18 18:34:28 2018 +0000

    Disable CUPS print backend for gtk4
    
    We don't have CUPS available on Continuous because of its dependency on
    Samba.

 manifest.json |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/manifest.json b/manifest.json
index ba35130..1f88b2f 100644
--- a/manifest.json
+++ b/manifest.json
@@ -624,7 +624,8 @@
                  "name": "gtk4",
                  "config-opts": ["-Ddocumentation=false",
                                  "-Dman-pages=false",
-                                 "-Dintrospection=true"],
+                                 "-Dintrospection=true",
+                                 "-Dprintbackends=file,lpr"],
                  "irc_room": "gtk+"},
 
                {"src": "fd:plymouth",