[gtkmm-documentation/kjellahl/testci] CI: Add docbook-style-xsl

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 545a651c99339863c590e2322289a7ed06522229
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sun Jan 9 11:33:03 2022 +0100

    CI: Add docbook-style-xsl

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 40a52c9..392bc30 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -21,6 +21,7 @@ docs-validation:
       itstool
       libxml2
       docbook5-schemas
+      docbook-style-xsl
       libxslt
       ninja-build
       python3