Fix compilation error for QRhi stereo manual test

Add a guard to prevent compilation error on
non-Windows platform

Change-Id: Ibe0d74c1dfe3d4656addeb80c702b6e254d37d84
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Kristoffer Skau <kristoffer.skau@qt.io>

tests/manual/rhi/stereo/window.cpp

@@ -93,6 +93,7 @@ void Window::init()
         m_rhi.reset(QRhi::create(QRhi::OpenGLES2, &params, rhiFlags));
         break;
     }
+#ifdef Q_OS_WIN
     case QRhi::D3D11:
     {
         QRhiD3D11InitParams params;
@@ -106,6 +107,7 @@ void Window::init()
         m_rhi.reset(QRhi::create(QRhi::D3D12, &params, rhiFlags));
         break;
     }
+#endif
     default:
         break;
     }