[gnome-backgrounds] fix build

[Jakub Steiner <jimmac src gnome org>]

commit 3d9374b4ae9694ba2facdfffa5bc56659a289b22
Author: Jakub Steiner <jimmac gmail com>
Date:   Thu May 12 01:44:49 2022 +0200

    fix build

 backgrounds/meson.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/backgrounds/meson.build b/backgrounds/meson.build
index a0a8e69..6e801ad 100644
--- a/backgrounds/meson.build
+++ b/backgrounds/meson.build
@@ -19,8 +19,8 @@ backgrounds = [
   'licorice-l.jpg',
   'pixels-d.png',
   'pixels-l.png',
-  'symbolic-d.webp',
-  'symbolic-l.webp',
+  'symbolic-d.jxl',
+  'symbolic-l.jxl',
   'truchet-d.jpg',
   'truchet-l.jpg',
   'vnc-d.png',