dllmain: exclude from Cygwin builds

dllmain: exclude from Cygwin builds

On Cygwin, it is unsafe to call POSIX functions from DllMain, which
OPENSSL_thread_stop does.  Additionally, it should be unnecessary as
OpenSSL uses pthread_key_create to register a thread destructor to do
thread cleanup in a POSIX way.

Reported-by: Yuyi Wang
Ref: https://cygwin.com/pipermail/cygwin/2025-June/258235.html

Fixes #17262
Closes https://github.com/curl/curl/pull/17528