Add missing cddlc install step

Signed-off-by: Alex Tzonkov <4975715+attzonko@users.noreply.github.com>

Author: attzonko

.github/workflows/validate-cddl.yml

@@ -19,6 +19,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y ruby
           sudo gem install cddl
+          sudo gem install cddlc
           sudo gem install cbor-diag
 
       - name: Convert DIAG to CBOR