Install cddl gem in dev image (#6521)

getting_started/setup_vm/roles/ccf_build/tasks/install.yml

@@ -57,3 +57,9 @@
   args:
     chdir: "{{ workspace }}/doxygen-{{ doxygen_ver }}"
   become: true
+
+- name: Install cddl checker
+  gem:
+    name: cddl
+    state: present
+  become: yes