Fixes sporadic failures in the Linux build tests.

Include the submodules in the git checkout, as sometimes the github
builder fails sometimes to update from the makefile.

.github/workflows/c-cpp.yml

@@ -13,6 +13,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+        submodules: 'true'
     - name: make
       run: make -j2
     - name: running testsuite