d3d11 demo: GetBuffer requires a void ** as 4th argument.

Fix warning with gcc.

demo/d3d11/main.c

@@ -94,7 +94,7 @@ set_swap_chain_size(int width, int height)
     desc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
     desc.ViewDimension = D3D11_RTV_DIMENSION_TEXTURE2D;
 
-    hr = IDXGISwapChain_GetBuffer(swap_chain, 0, &IID_ID3D11Texture2D, &back_buffer);
+    hr = IDXGISwapChain_GetBuffer(swap_chain, 0, &IID_ID3D11Texture2D, (void **)&back_buffer);
     assert(SUCCEEDED(hr));
 
     hr = ID3D11Device_CreateRenderTargetView(device, (ID3D11Resource *)back_buffer, &desc, &rt_view);