Win32: fixed build on Windows with OpenSSL 3.0.x (ticket #2379).

Win32: fixed build on Windows with OpenSSL 3.0.x (ticket #2379).

SSL_sendfile() expects integer file descriptor as an argument, but nginx
uses OS file handles (HANDLE) to work with files on Windows, and passing
HANDLE instead of an integer correctly results in build failure.  Since
SSL_sendfile() is not expected to work on Windows anyway, the code is now
disabled on Windows with appropriate compile-time checks.