GitHub: Recursively checkout submodules

This will become necessary when qlitehtml is a submodule with litehtml
as a sub-submodule.

Change-Id: I8a2ca47d24bc4b4f3ff88908a49d3f1933ec59ea
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

.github/workflows/build_cmake.yml

@@ -49,7 +49,7 @@ jobs:
     - name: Checkout submodules
       run: |
         git submodule set-url -- perfparser https://code.qt.io/qt-creator/perfparser.git
-        git submodule update --init
+        git submodule update --init --recursive
 
     - name: Download Ninja and CMake
       shell: cmake -P {0}