Define a missing variable.

defaults/main.yml

@@ -104,6 +104,7 @@ r_base_packages_java_el:
 r_base_packages_java_dev_el:
   - R-java-devel
 
+r_distribution_required_el_packages: []
 r_plugins_list_to_install: []
 #   - { name: 'R2WinBUGS', repo: '{{ r_cran_mirror_site }}' }
 #   - { name: 'R2jags', repo: '{{ r_cran_mirror_site }}' }