cmake: check USE_WINDOWS_SSPI when adding secur32 to CURL_LIBS

cmake: check USE_WINDOWS_SSPI when adding secur32 to CURL_LIBS

Instead of CURL_WINDOWS_SSPI.

When running CMake on Windows with no additional parameters (ie default
build configuration), the generated project files do not include the
`secur32.lib` library in the linker settings. This is because
the relevant check was looking at `CURL_WINDOWS_SSPI` instead of
`USE_WINDOWS_SSPI`.

`USE_WINDOWS_SSPI` is enabled when building with SChannel (the default
on Windows), or if `CURL_WINDOWS_SSPI` is specified on the command line.

Follow-up to 0d71b18153c8edb996738f8a362373fc72d0013b #17413

Closes #17728