use two-step casting to silence incompatible function type warning

[bashbaug]

Looks like a recent GitHub actions change caused our Windows CI builds to fail in the clangcl config due to -Werror and -Wcast-function-type-mismatch:

D:\a\OpenCL-ICD-Loader\OpenCL-ICD-Loader\loader\windows\icd_windows.c(284,54): error : cast from 'FARPROC' (aka 'long long (*)()') to 'PFN_CREATE_DXGI_FACTORY' (aka 'long (*)(const struct _GUID *const, void **)') converts to incompatible function type [-Werror,-Wcast-function-type-mismatch] [D:\a\OpenCL-ICD-Loader\OpenCL-ICD-Loader\build\OpenCL.vcxproj]

We can silence this warning with a two-step cast, e.g.

PFN_CREATE_DXGI_FACTORY pCreateDXGIFactory = (PFN_CREATE_DXGI_FACTORY)(void*)GetProcAddress(hDXGI, "CreateDXGIFactory");

We do this a couple of other places in the OpenCL ICD loader by casting through size_t, but I think casting through void* is more common. If desired though, I can switch to size_t, or turn the other casts to void* for consistency.

[CLAassistant]

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