Init submodules in CI build

Author: GDYendell

.github/workflows/cpp.yml

@@ -10,6 +10,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
+          submodules: 'recursive'
       - name: Build eiger-detector
         run: |
           git config --global --add safe.directory `pwd` && \