curl_threads: fix classic MinGW compile break

Classic MinGW still has _beginthreadex's return type as unsigned long
instead of uintptr_t [0]. uintptr_t is not even defined because of [1].

[0] https://sourceforge.net/p/mingw/mingw-org-wsl/ci/wsl-5.1-release/tree/mingwrt/include/process.h#l167
[1] https://sourceforge.net/p/mingw/mingw-org-wsl/ci/wsl-5.1-release/tree/mingwrt/include/process.h#l90

Bug: https://github.com/curl/curl/issues/2924#issuecomment-424334807
Closes https://github.com/curl/curl/pull/3051