Checkout submodules in CI pipeline.

Author: saivert

.github/workflows/flatpak.yml

@@ -16,6 +16,8 @@ jobs:
       options: --privileged
     steps:
     - uses: actions/checkout@v4
+      with:
+        submodules: 'true'
     - uses: flatpak/flatpak-github-actions/flatpak-builder@v6.3
       with:
         bundle: pwvucontrol.flatpak