[gtkmm-documentation] CI: Install the docbook5-schemas when validating

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 27cdbac83c22e9900796f3d97e96505f9cb1139f
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Fri Sep 24 17:35:05 2021 +0200

    CI: Install the docbook5-schemas when validating
    
    Necessary after the upgrade to DocBook 5.0.

 .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