[gtkmm-documentation/kjellahl/testci] CI: Add docbook5-schemas

[Kjell Ahlstedt <kjellahl src gnome org>]

commit a93a4e062c7d1156befc7608f34a7328fc268473
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Fri Sep 24 17:00:22 2021 +0200

    CI: Add docbook5-schemas

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 0c4404b..40a52c9 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -20,6 +20,7 @@ docs-validation:
       gettext
       itstool
       libxml2
+      docbook5-schemas
       libxslt
       ninja-build
       python3