Wrong path for SDL

Please include the following in your bug report:

**Version of emscripten/emsdk:**
```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.7-git (4af36cf80647f9a82be617a0ff32f3e56f220e41)
clang version 20.1.1 (https://github.com/llvm/llvm-project 424c2d9b7e4de40d0804dd374721e6411c27d1d1)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /mnt/HDExtra/bin/LLVM.d/bin
```

File `system/lib/cmake/SDL2/sdl2-config.cmake` has `set(SDL2_INCLUDE_DIRS "${EMSCRIPTEN_SYSROOT}/include/SDL2")`, but there is no directory `SDL2`, only `emscripten/system/include/SDL`