[gtk-doc] ci: add missing docbook package

[Stefan Sauer <stefkost src gnome org>]

commit 9d10022a0052c7ab936ce0b6a26d16b47120854f
Author: Stefan Sauer <ensonic users sf net>
Date:   Fri Dec 14 08:55:01 2018 +0100

    ci: add missing docbook package

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e5ecb96..5bd98d8 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -8,7 +8,7 @@ before_script:
   - apt update -qq
   - apt install -y -qq --no-install-recommends
                 autoconf automake build-essential libtool make meson pkg-config
-                docbook-xsl libxml2-utils xsltproc libglib2.0-dev
+                docbook docbook-xsl libxml2-utils xsltproc libglib2.0-dev
                 python3-lxml python3-parameterized python3-pip
                 python3-pygments python3-unittest2
   - export LANG=C.UTF-8