[gtk-doc] Add more deps to ci config

[Stefan Sauer <stefkost src gnome org>]

commit 01aa4d2d04b97ce47a84e33fa79ea8d0eb21e145
Author: Stefan Sauer <ensonic users sf net>
Date:   Thu Dec 13 21:57:23 2018 +0100

    Add more deps to ci config

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 681e907..5e026fe 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -3,7 +3,7 @@ image: debian:unstable
 before_script:
   - apt update -qq
   - apt install -y -qq autoconf automake build-essential libtool make pkg-config
-                       libxml2-utils xsltproc
+                       docbook-xsl libxml2-utils xsltproc
                        python3-lxml python3-parameterized python3-pip
                        python3-pygments python3-unittest2
   - export LANG=C.UTF-8