Backends: DX12: build fix for Clang. (#8582)

backends/imgui_impl_dx12.cpp

@@ -535,7 +535,7 @@ bool    ImGui_ImplDX12_CreateDeviceObjects()
                 return false;
         }
 
-        PFN_D3D12_SERIALIZE_ROOT_SIGNATURE D3D12SerializeRootSignatureFn = (PFN_D3D12_SERIALIZE_ROOT_SIGNATURE)::GetProcAddress(d3d12_dll, "D3D12SerializeRootSignature");
+        PFN_D3D12_SERIALIZE_ROOT_SIGNATURE D3D12SerializeRootSignatureFn = (PFN_D3D12_SERIALIZE_ROOT_SIGNATURE)(void*)::GetProcAddress(d3d12_dll, "D3D12SerializeRootSignature");
         if (D3D12SerializeRootSignatureFn == nullptr)
             return false;