CI: Install build-dependencies in 'build_reference'-CI-job

Since the update of 'serde_yaml' from 0.8 to 0.9,
the build-dependencies are required to complete this CI-job.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/624>

.gitlab-ci.yml

@@ -80,6 +80,7 @@ build_reference:
     paths:
       - _build/doc
   script:
+    - apt-get -y build-dep .
     - apt-get -y install cargo
     - cd _build
     - ../cargo.sh doc --no-deps --document-private-items --features 'zbus_v1_5,clap_v4'