CI: lv_sim: initialize libpng submodule for WITH_PNG=ON screenshot

InfiniSim PR InfiniTimeOrg/InfiniSim#10 adds
support for screenshots in png/bmp format using `libpng` submodule.
This new submodule must be added to the lv_sim workflow as well.

Author: NeroBurner

.github/workflows/lv_sim.yml

@@ -48,7 +48,7 @@ jobs:
       - name: Get InfiniSim repo
         run:  |
           git clone https://github.com/InfiniTimeOrg/InfiniSim.git --depth 1 --branch main
-          git -C InfiniSim submodule update --init lv_drivers
+          git -C InfiniSim submodule update --init lv_drivers libpng
 
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       # CMake