Fix Emscripten CI build

Emscripten was updated upstream with experimental SDL3 support. Set our CMake configuration to use SDL2 until the support for the next major version is more stable.

.github/workflows/build.yml

@@ -198,7 +198,8 @@ jobs:
         source emsdk-master/emsdk_env.sh
         emcmake cmake -B Build --preset samples -Wdev -Werror=dev -DCMAKE_BUILD_TYPE=Release \
         -DBUILD_SHARED_LIBS=OFF -DRMLUI_WARNINGS_AS_ERRORS=ON \
-        -DCMAKE_MODULE_PATH=$GITHUB_WORKSPACE/CMake/Modules/Emscripten
+        -DCMAKE_MODULE_PATH=$GITHUB_WORKSPACE/CMake/Modules/Emscripten \
+        -DRMLUI_SDL_VERSION_MAJOR=2
 
     - name: Build
       run: |-