gitlab-ci: install xmllint to validate 00-mesa-defaults.conf

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

.gitlab-ci/Dockerfile.ubuntu

@@ -161,5 +161,8 @@ RUN pip3 install mako
 # Use ccache to speed up builds
 RUN apt-get install -y ccache
 
+# We need xmllint to validate the XML files in Mesa
+RUN apt-get install -y libxml2-utils
+
 # Cleanup workdir
 WORKDIR /