[gtkmm] CI: Install libxml2-dev, required by subprojects

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 630f65c5832656ec5d391470b83ad5caec1b9046
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Tue Aug 16 09:18:56 2022 +0200

    CI: Install libxml2-dev, required by subprojects

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 05568d75..96694232 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -32,6 +32,7 @@ variables:
                    libgstreamer-plugins-bad1.0-dev
                    libgudev-1.0-dev
                    libsass-dev
+                   libxml2-dev
 
   GIO_EXTRA_MODULES: "/usr/lib/x86_64-linux-gnu/gio/modules"
   GIT_SUBMODULE_STRATEGY: normal