init gitmodules for flatpak test build

Author: NikolasK-source

network.koesling.test-modbus-tcp-client-shm.yml

@@ -10,6 +10,10 @@ modules:
   - name: Modbus_TCP_client_shm
     buildsystem: simple
     build-commands:
+    # init submodules
+    git submodule init
+    git submodule update
+
     # build
     - mkdir build
     - cmake -B build . -DCMAKE_BUILD_TYPE=Release -DCLANG_FORMAT=OFF -DCOMPILER_WARNINGS=OFF