[gimp-help] ci: add docbook-xml dependency

[Jacob Boerema <jboerema src gnome org>]

commit 822c717ef4243a5d290dc25aefb2142fe19f8348
Author: Jacob Boerema <jgboerema gmail com>
Date:   Thu Apr 1 14:17:09 2021 -0400

    ci: add docbook-xml dependency
    
    This fixes:
    I/O error : Attempt to load network entity http://www.docbook.org/xml/4.3/docbookx.dtd
    and
    warning: failed to load external entity "http://www.docbook.org/xml/4.3/docbookx.dtd";

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index d27790f64..00174a7c9 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -19,6 +19,7 @@ build-debian:
     - apt-get install -y --no-install-recommends
         automake
         build-essential
+        docbook-xml
         docbook-xsl
         pkg-config
         pngcrush