ci: Add ruby-devel installation to Rubocop job

Add a step on the GitHub Actions job "Rubocop" to install the package
'ruby-devel', which provide Ruby's headers, required by the 'racc'
(installed with 'bundle install').

Author: lcaparroz

.github/workflows/ci.yml

@@ -53,6 +53,9 @@ jobs:
       - name: Git Checkout
         uses: actions/checkout@v2
 
+      - name: Install ruby-devel package
+        run: zypper --non-interactive ruby-devel
+
       - name: Install project dependencies
         run: bundle install