Build failed with WITH_OPENCL option disabled (v6.16.1)
**To Reproduce**
```
cmake .. -G "Visual Studio 16 2019" -A "x64" -DXMRIG_DEPS="..\xmrig-deps\msvc2019\x64" -DWITH_OPENCL=OFF
cmake --build . --config Release
```
![error](https://user-images.githubusercontent.com/43396824/143965293-45971ef4-7aa7-4328-ac29-1e48a0974c7d.png)
